Load control device fo rhigh-efficiency loads

ABSTRACT

A two-wire load control device (such as, a dimmer switch) is operable to control the amount of power delivered from an AC power source to an electrical load (such as, a high-efficiency lighting load) and has substantially no minimum load requirement. The load control device includes a bidirectional semiconductor switch (such as a thyristor), which may be operable to remain conductive independent of the magnitude of a load current conducted through semiconductor switch and to conduct the load current to and from the load during a single half-cycle. The dimmer switch comprises a control circuit that conducts a control current through the load in order to generate a gate drive signal for rendering the bidirectional semiconductor switch conductive and non-conductive each half-cycle. The control circuit may provide a constant gate drive to the bidirectional semiconductor switch after the bidirectional semiconductor switch is rendered conductive each half-cycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 12/952,920, filed Nov. 23, 2010 by Robert C. Newman, Jr. et al. which is a non-provisional application of U.S. Provisional Patent Application No. 61/264,528, filed Nov. 25, 2009, and U.S. Provisional Patent Application No. 61/333,050, filed May 10, 2010, both entitled TWO-WIRE ANALOG DIMMER SWITCH FOR LOW-POWER LOADS, the entire disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to load control devices for controlling the amount of power delivered to an electrical load, and more particularly, to a two-wire analog dimmer switch for controlling the intensity of a low-power lighting load, such as a light-emitting diode (LED) light source having an LED driver circuit or a fluorescent lamp having an electronic dimming ballast.

2. Description of the Related Art

Prior art two-wire dimmer switches are coupled in series electrical connection between an alternating-current (AC) power source and a lighting load for controlling the amount of power delivered from the AC power source to the lighting load. A two-wire wall-mounted dimmer switch is adapted to be mounted to a standard electrical wallbox and comprises two load terminals: a hot terminal adapted to be coupled to the hot side of the AC power source and a dimmed hot terminal adapted to be coupled to the lighting load. In other words, the two-wire dimmer switch does not require a connection to the neutral side of the AC power source (i.e., the load control device is a “two-wire” device). Prior art “three-way” dimmer switches may be used in three-way lighting systems and comprise at least three load terminals, but do not require a connection to the neutral side of the AC power source.

The dimmer switch typically comprises a bidirectional semiconductor switch, e.g., a thryristor (such as a triac) or two field-effect transistors (FETs) in anti-series connection. The bidirectional semiconductor switch is coupled in series between the AC power source and the load and is controlled to be conductive and non-conductive for portions of a half-cycle of the AC power source to thus control the amount of power delivered to the electrical load. Generally, dimmer switches use either a forward phase-control dimming technique or a reverse phase-control dimming technique in order to control when the bidirectional semiconductor switch is rendered conductive and non-conductive to thus control the power delivered to the load. The dimmer switch may comprise a toggle actuator for turning the lighting load on and off and an intensity adjustment actuator for adjusting the intensity of the lighting load. Examples of prior art dimmer switches are described in greater detail is commonly-assigned U.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROL DEVICE; U.S. Pat. No. 6,969,959, issued Nov. 29, 2005, entitled ELECTRONIC CONTROL SYSTEMS AND METHODS; and U.S. Pat. No. 7,687,940, issued Mar. 30, 2010, entitled DIMMER SWITCH FOR USE WITH LIGHTING CIRCUITS HAVING THREE-WAY SWITCHES, the entire disclosures of which are hereby incorporated by reference.

With forward phase-control dimming, the bidirectional semiconductor switch is rendered conductive at some point within each AC line voltage half-cycle and remains conductive until approximately the next voltage zero-crossing, such that the bidirectional semiconductor switch is conductive for a conduction time each half-cycle. A zero-crossing is defined as the time at which the AC line voltage transitions from positive to negative polarity, or from negative to positive polarity, at the beginning of each half-cycle. Forward phase-control dimming is often used to control energy delivered to a resistive or inductive load, which may include, for example, an incandescent lamp or a magnetic low-voltage transformer. The bidirectional semiconductor switch of a forward phase-control dimmer switch is typically implemented as a thyristor, such as a triac or two silicon-controlled rectifiers (SCRs) coupled in anti-parallel connection, since a thyristor becomes non-conductive when the magnitude of the current conducted through the thyristor decreases to approximately zero amps.

Many forward phase-control dimmers include analog control circuits (such as timing circuits) for controlling when the thyristor is rendered conductive each half-cycle of the AC power source. The analog control circuit typically comprises a potentiometer, which may be adjusted in response to a user input provided from, for example, a linear slider control or a rotary knob in order to control the amount of power delivered to the lighting load. The analog control circuit is typically coupled in parallel with the thyristor and conducts a small timing current through the lighting load when the thyristor is non-conductive. The magnitude of the timing current is small enough such that the controlled lighting load is not illuminated to a level that is perceptible to the human eye when the lighting load is off.

Thyristors are typically characterized by a rated latching current and a rated holding current, and comprise two main terminals and a control terminal. The current conducted through the main terminals of the thyristor must exceed the latching current for the thyristor to become fully conductive. In addition, the current conducted through the main terminals of the thyristor must remain above the holding current for the thyristor to remain in full conduction. Since an incandescent lamp is a resistive lighting load, a typical forward phase-control dimmer switch is operable to conduct enough current through the incandescent lamp to exceed the rated latching and holding currents of the thyristor if the impedance of the incandescent lamp is low enough. Therefore, prior art forward phase-control dimmer switches are typically rated to operate appropriately with lighting loads having a power rating above a minimum power rating (e.g., approximately 40 W) to guarantee that the thyristor will be able to latch and remained latched when dimming the lighting load.

With reverse phase-control dimming, the bidirectional semiconductor switch is rendered conductive at the zero-crossing of the AC line voltage and rendered non-conductive at some point within each half-cycle of the AC line voltage, such that the bidirectional semiconductor switch is conductive for a conduction time each half-cycle. Reverse phase-control dimming is often used to control energy to a capacitive load, which may include, for example, an electronic low-voltage transformer. Since the bidirectional semiconductor switch must be rendered conductive at the beginning of the half-cycle, and must be able to be rendered non-conductive within the half-cycle, reverse phase-control dimming requires that the dimmer switch have two FETs in anti-serial connection, or the like. A FET is operable to be rendered conductive and to remain conductive independent of the magnitude of the current conducted through the FET. In other words, a FET is not limited by a rated latching or holding current as is a thyristor. However, prior art reverse phase-control dimmer switches have either required neutral connections and/or advanced control circuits (such as microprocessors) for controlling the operation of the FETs. In order to power a microprocessor, the dimmer switch must also comprise a power supply, which is typically coupled in parallel with the FETs. These advanced control circuits and power supplies add to the cost of prior art FET-based reverse phase-control dimmer switches (as compared to analog forward phase-control dimmer switches).

Further, in order to properly charge, the power supply of such a two-wire dimmer switch must develop an amount of voltage across the power supply and must conduct a charging current from the AC power source through the electrical load, in many instances even when the lighting load is off. If the power rating of the lighting load is too low, the charging current conducted by the power supply through the lighting load may be great enough to cause the lighting load to illuminate to a level that is perceptible to the human eye when the lighting load is off. Therefore, prior art FET-based reverse phase-control dimmer switches are typically rated to operate appropriately with lighting loads having a power rating above a minimum power rating to guarantee that the lighting load does not illuminate to a level that is perceptible to the human eye due to the power supply current when the lighting load is off. Some prior art load control devices, have included power supplies that only develop small voltages and draw small currents when charging, such that the minimum power rating of a controlling lighting load may be as low as 10 W. An example of such a power supply is described in greater detail in commonly-assigned U.S. patent application Ser. No. 12/751,324, filed Mar. 31, 2010, entitled SMART ELECTRONIC SWITCH FOR LOW-POWER LOADS, the entire disclosure of which is hereby incorporated by reference.

Nevertheless, it is desirable to be able to control the amount of power to electrical loads having power rating lower than those able to be controlled by the prior art forward and reverse phase-control dimmer switches. In order to save energy, high-efficiency lighting loads, such as, for example, compact fluorescent lamps (CFLs) and light-emitting diode (LED) light sources, are being used in place of or as replacements for conventional incandescent or halogen lamps. High-efficiency light sources typically consume less power and provide longer operational lives as compared to incandescent and halogen lamps. In order to illuminate properly, a load regulation device (e.g., such as an electronic dimming ballast or an LED driver) must be coupled between the AC power source and the respective high-efficiency light source (i.e., the compact fluorescent lamp or the LED light source) for regulating the power supplied to the high-efficiency light source.

A dimmer switch controlling a high-efficiency light source may be coupled in series between the AC power source and the load control device for the high-efficiency light source. Some high-efficiency lighting loads are integrally housed with the load regulation devices in a single enclosure. Such an enclosure may have a screw-in base that allows for mechanical attachment to standard Edison sockets and provide electrical connections to the neutral side of the AC power source and either the hot side of the AC power source or the dimmed-hot terminal of the dimmer switch (e.g., for receipt of the phase-control voltage). The load regulation circuit is operable to control the intensity of the high-efficiency light source to the desired intensity in response to the conduction time of the bidirectional semiconductor switch of the dimmer switch.

However, the load regulation devices for the high-efficiency light sources may have high input impedances or input impedances that vary in magnitude throughout a half-cycle. Therefore, when a prior-art forward phase-control dimmer switch is coupled between the AC power source and the load regulation device for the high-efficiency light source, the load control device may not be able to conduct enough current to exceed the rated latching and/or holding currents of the thyristor. In addition, when a prior-art reverse phase-control dimmer switch is coupled between the AC power source and the load regulation device, the magnitude of the charging current of the power supply may be great enough to cause the load regulation device to illuminate the controlled high-efficiency light source to a level that is perceptible by the human eye when the light source should be off.

The impedance characteristics of the load regulation device may negatively affect the magnitude of the phase-control voltage received by the load regulation device, such that the conduction time of the received phase-control voltage is different from the actually conduction time of the bidirectional semiconductor switch of the dimmer switch (e.g., if the load regulation device has a capacitive impedance). Therefore, the load regulation device may control the intensity of the high-efficiency light source to an intensity that is different than the desired intensity as directed by the dimmer switch. In addition, the charging current of the power supply of the dimmer switch may build up charge at the input of a load regulation device having a capacitive input impedance, thus negatively affecting the low-end intensity that may be achieved.

Therefore, there exists a need for a two-wire load control device that may be coupled between an AC power source and a load regulation device for a high-efficiency light source and is able to properly control the intensity of the high-efficiency light source.

SUMMARY OF THE INVENTION

As described herein, a two-wire load control device (such as, a dimmer switch) is able to control the amount of power delivered from an AC power source to an electrical load (such as, a high-efficiency lighting load) and has substantially no minimum load requirement. The dimmer switch includes a bidirectional semiconductor switch that is adapted to be coupled in series electrical connection between the AC power source and the electrical load for conducting a load current from the AC power source to the electrical load. The bidirectional semiconductor switch has a control input for rendering the bidirectional semiconductor switch conductive and non-conductive. The bidirectional semiconductor switch is operable to be rendered conductive each half-cycle and to remain conductive independent of the magnitude of the load current conducted through bidirectional semiconductor switch.

The dimmer switch also comprises a digital control circuit that receives a signal representative of a voltage developed across the bidirectional semiconductor switch, and is operable to determine a half-cycle start time near the beginning of a half-cycle of the AC power source in response to the signal representative of the voltage developed across the bidirectional semiconductor switch. The digital control circuit comprises a power supply that generates a supply voltage and conducts a charging current through the load for generating the supply voltage. The digital control circuit generates a gate drive signal that is operatively coupled to the control input of the bidirectional semiconductor switch, drives the gate drive signal to a first magnitude to render the bidirectional semiconductor switch conductive after a first variable amount of time has elapsed since the half-cycle start time, and maintains the gate drive signal at the first magnitude after the bidirectional semiconductor switch is rendered conductive, such that the bidirectional semiconductor switch remains conductive independent of the magnitude of the load current conducted through the bidirectional semiconductor switch. The digital control circuit drives the gate drive signal to a second magnitude to render the bidirectional semiconductor switch non-conductive after a second fixed amount of time has elapsed since the half-cycle start time. The digital control circuit further controls the second fixed amount of time to be approximately equal during each half-cycle of the AC power source, and varies the first variable amount of time in response to the desired amount of power to be delivered to the load to thus control the amount of power delivered to the load to the desired amount.

As further described herein, a load control device for controlling the amount of power delivered from an AC power source to an electrical load (such as a high-efficiency lighting load including a high-efficiency light source and a load regulation device) comprises a thyristor and a control circuit. The thyristor is adapted to be coupled in series electrical connection between the AC power source and the load for conducting a load current from the AC power source to the load. The control circuit generates a drive signal for rendering the thyristor conductive at a firing time during each half-cycle of the AC power source. The control circuit is operable to maintain the thyristor conductive after the firing time, such as the thyristor is able to remain conductive independent of the magnitude of the load current conducted through the thyristor, and to conduct the load current to and from the load during a single half-cycle of the AC power source.

Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the following detailed description with reference to the drawings in which:

FIG. 1 is a simplified block diagram of a lighting control system including a two-wire analog dimmer switch for controlling the intensity of an LED light source according to a first embodiment of the present invention;

FIG. 2 is a simplified block diagram of the dimmer switch of FIG. 1 according to the first embodiment of the present invention;

FIGS. 3A and 3B show example waveforms illustrating the operation of the dimmer switch of FIG. 1 according to the first embodiment of the present invention;

FIG. 4 is a simplified schematic diagram of the dimmer switch of FIG. 2 according to the first embodiment of the present invention;

FIG. 5 is a simplified schematic diagram of a timing circuit of the dimmer switch of FIG. 2;

FIG. 6 is a simplified schematic diagram of a dimmer switch according to a second embodiment of the present invention;

FIG. 7 is a simplified block diagram of a reverse-phase control dimmer switch according to a third embodiment of the present invention;

FIG. 8 is a simplified timing diagram showing examples of waveforms illustrating the operation of the dimmer switch of FIG. 7 according to the third embodiment of the present invention;

FIG. 9 is a simplified schematic diagram of the dimmer switch of FIG. 7 according to the third embodiment of the present invention;

FIG. 10 is a simplified schematic diagram of a dimmer switch according to a fourth embodiment of the present invention;

FIG. 11 is a simplified schematic diagram of a dimmer switch according to a fifth embodiment of the present invention;

FIG. 12 is a simplified timing diagram showing examples of waveforms illustrating the operation of the dimmer switch of FIG. 11 according to the fifth embodiment of the present invention;

FIG. 13 is a simplified schematic diagram of a dimmer switch according to a sixth embodiment of the present invention;

FIG. 14 is a simplified schematic diagram of a dimmer switch according to a seventh embodiment of the present invention;

FIG. 15 is a simplified schematic diagram of a dimmer switch according to an eighth embodiment of the present invention;

FIG. 16 is a simplified schematic diagram of a dimmer switch having a digital control circuit according to a ninth embodiment of the present invention;

FIG. 17 is a simplified flowchart of a switch procedure executed by a microprocessor of the dimmer switch of FIG. 16 according to the ninth embodiment of the present invention; and

FIG. 18 is a simplified flowchart of a control procedure periodically executed by the microprocessor of the dimmer switch of FIG. 16 according to the ninth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.

FIG. 1 is a simplified block diagram of a lighting control system 10 including a “two-wire” dimmer switch 100 for controlling the amount of power delivered to a high-efficiency lighting load 101 including a load regulation device, e.g., a light-emitting diode (LED) driver 102, and a high-efficiency light source, e.g., an LED light source 104 (or “light engine”). The dimmer switch 100 has a hot terminal H coupled to an alternating-current (AC) power source 105 for receiving an AC mains line voltage V_(AC), and a dimmed-hot terminal DH coupled to the LED driver 102. The dimmer switch 100 does not require a direct connection to the neutral side N of the AC power source 105. The dimmer switch 100 generates a phase-control voltage V_(PC) (e.g., a dimmed-hot voltage) at the dimmed-hot terminal DH and conducts a load current I_(LOAD) through the LED driver 102. The dimmer switch 100 may either use forward phase-control dimming or reverse phase-control dimming techniques to generate the phase-control voltage V_(PC).

As defined herein, a “two-wire” dimmer switch or load control device does not require a require a direct connection to the neutral side N of the AC power source 105. In other words, all currents conducted by the two-wire dimmer switch must also be conducted through the load. A two-wire dimmer switch may have only two terminals (i.e., the hot terminal H and the dimmed hot terminal DH as shown in FIG. 1). Alternatively, a two-wire dimmer switch (as defined herein) could comprise a three-way dimmer switch that may be used in a three-way lighting system and has at least three load terminals, but does not require a neutral connection. In addition, a two-wire dimmer switch may comprise an additional connection that provides for communication with a remote control device (for remotely controlling the dimmer switch), but does not require the dimmer switch to be directly connected to neutral.

The LED driver 102 and the LED light source 104 may be both included together in a single enclosure, for example, having a screw-in base adapted to be coupled to a standard Edison socket. When the LED driver 102 is included with the LED light source 104 in the single enclosure, the LED driver only has two electrical connections: to the dimmer switch 100 for receiving the phase-control voltage V_(PC) and to the neutral side N of the AC power source 105. The LED driver 102 comprises a rectifier bridge circuit 106 that receives the phase-control voltage V_(PC) and generates a bus voltage V_(BUS) across a bus capacitor C_(BUS). The LED driver 102 further comprises a load control circuit 107 that receives the bus voltage V_(BUS) and controls the intensity of the LED light source 104 in response to the phase-control signal V_(PC). Specifically, the load control circuit 107 of the LED driver 102 is operable to turn the LED light source 104 on and off and to adjust the intensity of the LED light source to a target intensity L_(TRGT) (i.e., a desired intensity) in response to the phase-control signal V_(PC). The target intensity L_(TRGT) may range between a low-end intensity L_(LE) (e.g., approximately 1%) and a high-end intensity L_(HE) (e.g., approximately 100%). The LED driver 102 may also comprise a filter network 108 for preventing noise generated by the load control circuit 107 from being conducted on the AC mains wiring. An example of the LED driver 102 is described in greater detail in U.S. patent application Ser. No. 12/813,908, filed Jun. 11, 2009, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which is hereby incorporated by reference.

In addition, the LED driver 102 may comprise an artificial load circuit 109 for conducting current (in addition to the load current I_(LOAD)) through the dimmer switch 100. Accordingly, if the dimmer switch 100 includes a triac for generating the phase-control voltage V_(PC), the artificial load circuit 109 may conduct enough current to ensure that the magnitude of the total current conducted through the triac of the dimmer switch 100 exceeds the rated latching and holding currents of the triac. In addition, the artificial load circuit 109 may conduct a timing current if the dimmer switch 100 comprises a timing circuit and may conduct a charging current if the dimmer switch comprises a power supply, such that these currents need not be conducted through the load control circuit 107 and do not affect the intensity of the LED light source 104.

The artificial load circuit 109 may simply comprise a constant impedance circuit (e.g., a resistor) or may comprise a current source circuit. Alternatively, the artificial load circuit 109 may be controllable, such that the artificial load circuit may be enabled and disabled to thus selectively conduct current through the dimmer switch 100. In addition, the artificial load circuit 109 may be controlled to conduct different amounts of current depending upon the magnitude of the AC mains line voltage V_(AC), the present time during a half-cycle of the AC mains line voltage, or the present operating mode of the LED driver 102. Examples of artificial load circuits are described in greater detail in commonly-assigned U.S. patent application Ser. No. 12/438,587, filed Aug. 5, 2009, entitled VARIABLE LOAD CIRCUITS FOR USE WITH LIGHTING CONTROL DEVICES, and U.S. patent application Ser. No. 12/950,079, filed Nov. 19, 2010, entitled CONTROLLABLE-LOAD CIRCUIT FOR USE WITH A LOAD CONTROL DEVICE, the entire disclosures of which are hereby incorporated by reference.

Alternatively, the high-efficiency light source could comprise a compact fluorescent lamp (CFL) and the load regulation device could comprise an electronic dimming ballast. In addition, the dimmer switch 100 could alternatively control the amount of power delivered to other types of electrical loads, for example, by directly controlling a lighting load or a motor load. An example of a screw-in light source having a fluorescent lamp and an electronic dimming ballast is described in greater detail in U.S. patent application Ser. No. 12/704,781, filed Feb. 12, 2010, entitled HYBRID LIGHT SOURCE, the entire disclosure of which is hereby incorporated by reference.

The dimmer switch 100 comprises a user interface having a rocker switch 116 and an intensity adjustment actuator 118 (e.g., a slider knob as shown in FIG. 1). The rocker switch 116 allows for turning on and off the LED light source 104, while the intensity adjustment actuator 118 allows for adjustment of the target intensity L_(TRGT) of the LED light source 104 from the low-end intensity L_(LE) to the high-end intensity L_(HE). Examples of user interfaces of dimmer switches are described in greater detail in commonly-assigned U.S. patent application Ser. No. 12/363,258, filed Jan. 30, 2009, entitled LOAD CONTROL DEVICE HAVING A VISUAL INDICATION OF ENERGY SAVINGS AND USAGE INFORMATION, the entire disclosure of which is hereby incorporated by reference.

FIG. 2 is a simplified block diagram of the dimmer switch 100 according to a first embodiment of the present invention. FIGS. 3A and 3B show example waveforms illustrating the operation of the dimmer switch 100 according to the first embodiment of the present invention. The dimmer switch 100 comprises a bidirectional semiconductor switch 110 coupled between the hot terminal H and the dimmed hot terminal DH for generating the phase-control voltage V_(PC) (as shown in FIGS. 3A and 3B) and controlling of the amount of power delivered to the LED driver 102. The bidirectional semiconductor switch 110 comprises a control input (e.g., a gate), which may receive control signals for rendering the bidirectional semiconductor switch conductive and non-conductive. The bidirectional semiconductor switch 110 may comprise a single device, such as a triac, or a combination of devices, such as, two field-effect transistors (FETs) coupled in anti-series connection. According to the first embodiment of the present invention, the phase-control voltage V_(PC) comprises a forward phase-control voltage. In other words, the phase-control voltage V_(PC) has a magnitude of approximately zero volts at the beginning of each half-cycle during a non-conduction time T_(NC), and has a magnitude equal to approximately the magnitude of the AC line voltage V_(AC) of the AC power source 105 during the rest of the half-cycle, i.e., during a conduction time T_(CON).

The dimmer switch 100 comprises a mechanical air-gap switch S112 electrically coupled to the hot terminal H and in series with the bidirectional semiconductor switch 110, such that the LED light source 104 is turned off when the switch is open. When the air-gap switch S112 is closed, the dimmer switch 100 is operable to control the bidirectional semiconductor switch 110 to control the amount of power delivered to the LED driver 102. The air-gap switch S112 is mechanically coupled to the rocker switch 116 of the user interface of the dimmer switch 100, such that the switch may be opened and closed in response to actuations of the rocker switch. The dimmer switch 100 further comprises a rectifier circuit 114 coupled across the bidirectional semiconductor switch 110 and operable to generate a rectified voltage V_(RECT) (i.e., a signal representative of the voltage developed across the bidirectional semiconductor switch).

According to the first embodiment, the dimmer switch 100 comprises an analog control circuit 115 including a power supply 120, a constant-rate one-shot timing circuit 130, and a variable-threshold trigger circuit 140 (i.e., a gate drive circuit). The control circuit 115 receives the rectified voltage V_(RECT) from the rectifier circuit 114 and conducts a control current I_(CNTL) through the load (i.e., the LED driver 102) in order to generate a drive voltage V_(DR) for controlling the bidirectional semiconductor switch 110 to thus adjust the intensity of the LED light source 104 in response to the intensity adjustment actuator 118. The power supply 120 of the control circuit 115 conducts a charging current I_(CHRG) through the LED driver 102 in order to generate a supply voltage V_(CC) (e.g., approximately 11.4 volts). The charging current I_(CHRG) of the power supply makes up a portion of the control current I_(CNTL) of the control circuit 115.

The timing circuit 130 receives the supply voltage V_(CC) and generates a timing voltage V_(TIM) (i.e., a timing signal), which comprises a ramp signal having a constant rate of increasing magnitude (i.e., a constant positive slope) as shown in FIGS. 3A and 3B. When the bidirectional semiconductor switch 110 is non-conductive at the beginning of each half-cycle, the timing circuit 130 also receives the rectified voltage V_(RECT) and is able to derive zero-crossing timing information from the voltage developed across the LED driver 102 (i.e., from the control current I_(CNTL) conducted through the LED driver 102). The timing voltage V_(TIM) begins increasing from approximately zero volts shortly after the zero-crossings of the AC line voltage V_(AC) (i.e., shortly after the beginning of each half-cycle as shown at times t₁, t₄ in FIGS. 3A and 3B) and continues increasing at the constant rate. After a fixed amount of time T_(TIM) has elapsed since the timing voltage V_(TIM) started increasing from zero volts during the present half-cycle, the timing voltage V_(TIM) is driven to approximately zero volts near the next zero-crossing (i.e., near the end of the present half-cycle as shown at time t₃ in FIGS. 3A and 3B). Since the timing voltage V_(TIM) increases in magnitude at the constant rate for the fixed amount of time T_(TIM) each half-cycle, the timing voltage V_(TIM) is essentially identical during each half-cycle as shown in FIGS. 3A and 3B.

Referring back to FIG. 2, the variable-threshold trigger circuit 140 receives the timing voltage V_(TIM) from the timing circuit 130, and generates a drive voltage V_(DR) (i.e., a gate drive voltage) for controlling the bidirectional semiconductor switch 110 to thus adjust the intensity of the LED light source 104 in response to actuations of the intensity adjustment actuator 118. The trigger circuit 140 is characterized by a variable threshold (i.e., a variable threshold voltage V_(TH) shown in FIGS. 3A and 3B) that may be adjusted in response to the intensity adjustment actuator 118 of the user interface of the dimmer switch 100.

A gate coupling circuit 150 couples the drive voltage V_(DR) to the gate of the bidirectional semiconductor switch 110 for thus rendering the bidirectional semiconductor switch 110 conductive and non-conductive in response to the magnitude of the variable threshold voltage V_(TH). When the magnitude of the timing voltage V_(TIM) exceeds the magnitude of a variable threshold voltage V_(TH) each half-cycle (as shown at firing times t₂, t₅ in FIGS. 3A and 3B), the trigger circuit 140 is operable to drive the drive signal V_(DR) to a first magnitude (e.g., approximately zero volts as shown in FIGS. 3A and 3B) to thus render the bidirectional semiconductor switch 110 conductive each half-cycle (as will be described in greater detail below with reference to FIG. 4). The drive signal V_(DR) is then driven to a second magnitude (e.g., approximately the supply voltage V_(CC) as shown in FIGS. 3A and 3B) to render the bidirectional semiconductor switch 110 non-conductive when the timing voltage V_(TIM) is controlled to approximately zero volts shortly before the next zero-crossing. The variable threshold voltage V_(TH) is shown at two different magnitudes in FIGS. 3A and 3B, which results in the drive signal V_(DR) being driven low to zero volts (and thus rendering the bidirectional semiconductor switch 110 conductive) for different amounts of time.

As shown in FIGS. 3A and 3B, the control circuit 115 of the dimmer switch 100 is operable to provide a constant gate drive to the bidirectional semiconductor switch 110 by maintaining the drive voltage V_(DR) low for the remainder of the half-cycle after the bidirectional semiconductor switch 110 is rendered conductive (as shown at firing times t₂, t₅). Accordingly, the bidirectional semiconductor switch 110 will remain conductive independent of the magnitude of the load current I_(LOAD) conducted through the bidirectional semiconductor switch and the LED driver 102. When the bidirectional semiconductor switch 110 is conductive and the magnitude of the phase control voltage V_(PC) is greater than approximately the magnitude of the bus voltage V_(BUS) of the LED driver 102, the LED driver 102 will begin to conduct the load current I_(LOAD) through the bidirectional semiconductor switch. Since the bus capacitor C_(BUS) of the LED driver 102 may charge quickly, the magnitude of the load current I_(LOAD) may quickly peak before subsiding down to a substantially small magnitude (e.g., approximately zero amps). As previously mentioned, the bidirectional semiconductor switch 110 will remain conductive independent of the magnitude of the load current I_(LOAD) because the control circuit 115 is providing constant gate drive to the bidirectional semiconductor switch. In addition to quickly increasing and decreasing in magnitude, the load current I_(LOAD) may also change direction after the bidirectional semiconductor switch 110 is rendered conductive. Therefore, the bidirectional semiconductor switch 110 is also operable to conduct current in both directions (i.e., to and from the LED driver 102) after the bidirectional semiconductor switch is rendered conductive during a single half-cycle, thereby allowing any capacitors in the filter network 108 of the LED driver 102 to follow the magnitude of the AC line voltage V_(AC) of the AC power source 105.

FIG. 4 is a simplified schematic diagram of the dimmer switch 100. As shown in FIG. 4, the bidirectional semiconductor switch 110 of the dimmer switch 100 of the first embodiment is implemented as a triac 110′, but may alternatively be implemented as one or more silicon-controlled rectifiers (SCRs), or any suitable thyristor. While not shown in FIG. 4, a choke inductor may be coupled in series with the triac 110′, and a filter circuit (such as a filter capacitor) may be coupled between the hot terminal H and the dimmed hot terminal DH (i.e., in parallel with the triac) to prevent noise generated by the switching of the triac from being conducted on the AC mains wiring. The rectifier circuit 114 comprises a full-wave rectifier bridge having four diodes D114A, D114B, D114C, D114D. The rectifier bridge of the rectifier circuit 114 has AC terminals coupled in series between the hot terminal H and the dimmed hot terminal DH, and DC terminals for providing the rectified voltage V_(RECT) to the timing circuit 130 when the triac 110′ is non-conductive and a voltage is developed across the dimmer switch 100. The control circuit 115 conducts the control current I_(CNTL) through the rectifier circuit 114 and the LED driver 102. Accordingly, the total current conducted through the LED driver 102 each half-cycle is the sum of the load current I_(LOAD) conducted through the bidirectional semiconductor switch 110, the control current I_(CNTL) conducted through the control circuit 115 of the dimmer switch 100, and any leakage current conducted through the filter circuit (that may be coupled between the hot terminal H and the dimmed hot terminal DH).

As shown in FIG. 4, the power supply 120 comprises, for example, a pass-transistor circuit that generates the supply voltage V_(CC). The pass-transistor circuit comprises an NPN bipolar junction transistor Q122 having a collector coupled to receive the rectifier voltage V_(RECT) through a resistor R124 (e.g., having a resistance of approximately 100 kΩ). The base of the transistor Q122 is coupled to the rectifier voltage V_(RECT) through a resistor R125 (e.g., having a resistance of approximately 150 kΩ), and to circuit common through a zener diode Z126 (e.g., having a break-over voltage of approximately 12 volts). The power supply 120 further comprises a storage capacitor C128, which is able to charge through the transistor Q122 to a voltage equal to approximately the break-over voltage of the zener diode Z126 minus the base-emitter drop of the transistor Q122. The storage capacitor C128 has, for example, a capacitance of approximately 10 μF, and operates to maintain the supply voltage V_(CC) at an appropriate magnitude (i.e., approximately 11.4 volts) to allow the timing circuit 120 to generate the timing voltage V_(TIM) and the gate coupling circuit 150 to continue rendering the triac 110′ conductive after the firing times each half-cycle.

The timing circuit 130 comprises a constant ramp circuit 160, a one-shot latch circuit 170, and a reset circuit 180. The constant ramp circuit 160 receives the supply voltage V_(CC) and causes the timing voltage V_(TIM) to increase in magnitude at the constant rate. The reset circuit 180 receives the rectified voltage V_(RECT) and is coupled to the timing voltage V_(TIM), such that the reset circuit is operable to start the timing voltage V_(TIM) increasing in magnitude from approximately zero volts shortly after the beginning of each half-cycle at a half-cycle start time (e.g., times t₁, t₄ in FIGS. 3A and 3B). Specifically, the reset circuit 180 is operable to enable the timing voltage V_(TIM) (i.e., to start the increase of the magnitude of the timing voltage V_(TIM)) in response to a positive-going transition of the rectified voltage V_(RECT) across a reset threshold V_(RST) that remains above the reset threshold V_(RST) for at least a predetermined amount of time. The one-shot latch circuit 170 provides a latch voltage V_(LATCH) to the reset circuit 180 to prevent the reset circuit 180 from resetting the timing voltage V_(TIM) until the end of the half-cycle, thus ensuring that the reset circuit only restarts the generation of the timing voltage once each half-cycle.

The one-shot latch circuit 170 stops the generation of the timing voltage V_(TIM) by controlling the magnitude of the timing voltage V_(TIM) to approximately 0.6 volts at the end of the fixed amount of time from when the reset circuit 180 enabled the timing voltage V_(TIM) (e.g., near the end of the half-cycle at time t₃ in FIGS. 3A and 3B). After the one-shot latch circuit 170 controls the magnitude of the timing voltage V_(TIM) to approximately 0.6 volts, the reset circuit 180 is once again able to enable the generation of the timing voltage V_(TIM) after the beginning of the next half-cycle (i.e., at time t₄ in FIGS. 3A and 3B). As a result, a dead time T_(DT) exists between the time when the one-shot latch circuit 170 drives the timing voltage V_(TIM) to approximately 0.6 volts and the reset circuit 180 enables the generation of the timing voltage V_(TIM) by controlling the magnitude of the timing voltage V_(TIM) down to approximately zero volts.

The variable-threshold trigger circuit 140 comprises a comparator U142 having an inverting input that receives the timing voltage V_(TIM) from the timing circuit 130. The variable-threshold trigger circuit 140 also comprises a potentiometer R144 that is mechanically coupled to the slider knob of the intensity adjustment actuator 118. The potentiometer R144 has a resistive element coupled between the supply voltage V_(CC) and circuit common and a wiper terminal that generates the variable threshold voltage V_(TH). The variable threshold voltage V_(TH) comprises a DC voltage that varies in magnitude in response to the position of the slider knob of the intensity adjustment actuator 118 and is provided to a non-inverting input of the comparator U142. The drive voltage V_(DR) is generated at an output of the comparator U142 and is provided to the gate coupling circuit 150 for rendering the triac 110′ conductive and non-conductive. The gate coupling circuit 150 comprises an opto-coupler U152 having an input photodiode, which is coupled between the supply voltage V_(CC) and the output of the comparator U142 and in series with a resistor R154 (e.g., having a resistance of approximately 8.2 kΩ). The opto-coupler U152 has an output phototriac that is coupled between the hot terminal H and the gate of the triac 110′ and in series with a resistor R156 (e.g., having a resistance of approximately 100 Ω).

When the magnitude of the timing voltage V_(TIM) is below the magnitude of the variable threshold voltage V_(TH), the magnitude of the drive voltage V_(DR) at the output of the comparator U142 of the variable-threshold trigger circuit 140 remains high at approximately the supply voltage V_(CC), such that the triac 110′ remains non-conductive. When the magnitude of the timing voltage V_(TIM) increases above the variable threshold voltage V_(TH), the comparator U142 drives the drive voltage V_(DR) low to approximately circuit common, such that the input photodiode of the opto-coupler U152 conducts a drive current I_(DR). As a result, the output phototriac of the opto-coupler U152 is rendered conductive, thus also rendering the triac 110′ conductive. Accordingly, the drive voltage V_(DR) is driven low to render the triac 110′ conductive after a variable amount of time has elapsed since the half-cycle start time (i.e., the non-conduction time T_(NC) as shown in FIGS. 3A and 3B), where the variable amount of time is adjusted in response to intensity adjustment actuator 118 and the variable threshold voltage V_(TH). After the triac 110′ is rendered conductive each half-cycle, the timing circuit 130 continues to generate the timing voltage V_(TIM). Thus, the magnitude of the timing voltage V_(TIM) remains above the variable threshold voltage V_(TH) and the triac 110′ remains conductive until approximately the end of the half-cycle when the one-shot latch circuit 170 drives the timing voltage to approximately zero volts.

According to the first embodiment of the present invention, the latch circuit 170 is operable to control the timing voltage V_(TIM) to approximately zero volts (thus controlling the magnitude of the drive voltage V_(DR) high to approximately the supply voltage V_(CC)) shortly before the end of the present half-cycle (as shown at time t₃ in FIGS. 3A and 3B). Accordingly, the length of the timing voltage V_(TIM) (i.e., the fixed amount of time T_(TIM)) is slightly smaller than the length T_(HC) of each half-cycle. The dead time T_(DT) (or “blanking pulse”) in the timing voltage V_(TIM) at the end of the half-cycle allows the triac 110′ to commutate off (i.e., become non-conductive) when the magnitude of the load current I_(LOAD) through the triac reduces to approximately zero amps at the end of the half-cycle.

Because the LED driver 102 may have a capacitive input impedance, the magnitude of the phase-control voltage V_(PC) may not quickly decrease to zero volts near the zero-crossing of the AC mains lines voltage VAC after the triac 110′ becomes non-conductive at the end of each half-cycle. Therefore, according to the first embodiment of the present invention, the reset circuit 180 only starts the timing voltage V_(TIM) after a zero-crossing of the AC mains lines voltage V_(AC), i.e., in response to the magnitude of the rectified voltage V_(RECT) exceeding the reset threshold V_(RST) when the rectified voltage is increasing in magnitude. The reset circuit 180 is prevented from resetting the timing voltage V_(TIM) in response to the magnitude of the rectified voltage V_(RECT) dropping below the reset threshold V_(RST), which may or may not happen each half-cycle due to the capacitive input impedance of the LED driver 102.

FIG. 5 is a simplified schematic diagram of the timing circuit 130. The constant ramp circuit 160 receives the supply voltage V_(CC) and generates the timing voltage V_(TIM) across a timing capacitor C162 (e.g., having a capacitance of approximately 50 nF). The constant ramp circuit 160 comprises a constant current source for conducting a constant timing current I_(TIM) through the timing capacitor C162, such that the timing voltage V_(TIM) has a constant slope. The constant current source circuit comprises a PNP bipolar junction transistor Q164 having an emitter coupled to the supply voltage V_(CC) via a resistor R165 (e.g. having a resistance of approximately 10 kΩ). Two diodes D166, D168 are coupled in series between the supply voltage V_(CC) and the base of the transistor Q164. A resistor R169 is coupled between the base of the transistor Q164 and circuit common and has, for example, a resistance of approximately 51 kΩ. A voltage having a magnitude of approximately the forward voltage drop of the diode D166 (e.g., approximately 0.6 V) is produced across the resistor R165, such that the resistor conducts the constant timing current I_(TIM) (e.g., approximately 70 μA) into the capacitor C162. The rate at which the magnitude of the timing voltage V_(TIM) increases with respect to time (i.e., dV_(TIM)/dt) is a function of the magnitude of the timing current I_(TIM) and the capacitance C_(C162) of the capacitor C162 (i.e., dV_(TIM)/dt=I_(TIM)/C₁₆₂), and may be equal to, for example, approximately 1.4 V/msec.

The one-shot latch circuit 170 comprises a comparator U172 having an inverting input coupled to the timing voltage V_(TIM). The timing voltage V_(TIM) is further coupled to an output of the comparator U172 via a diode D174. The one-shot latch circuit 170 includes a resistive divider, which is coupled in series electrical connection between the supply voltage V_(CC) and circuit common, and comprises two resistors R175, R176 having, for example, resistances of approximately 100 kΩ and 1 MΩ, respectively. The junction of the two resistors R175, R176 produces a latch threshold voltage V_(TH-L), which is provided to a non-inverting input of the comparator U172. The non-inverting input of the comparator U172 is also coupled to the output via a resistor R178 (e.g., having a resistance of approximately 1 kΩ). The latch voltage V_(LATCH) is generated at the output of the comparator U172 and is provided to the reset circuit 180 as will be described in greater detail below.

The reset circuit 180 comprises a first comparator U181 having a non-inverting input that receives the rectified voltage V_(RECT) via the series combination of a zener diode Z182 and a resistor R183 (e.g., having a resistance of approximately 100 kΩ). The parallel combination of a capacitor C184 (e.g., having a capacitance of approximately 1000 pF) and a resistor R185 (e.g., having a resistance of approximately 20 kΩ) is coupled between the non-inverting input of the comparator U181 and circuit common. A zener diode Z186 (e.g., having a break-over voltage of approximately 12 volts) clamps the magnitude of the voltage produced between the non-inverting input of the comparator U181 and circuit common. The reset circuit 180 further comprises a resistive divider that has two resistors R187, R188 (e.g., having resistances of approximately 150 kΩ and 100 kΩ, respectively), and is coupled in series electrical connection between the supply voltage V_(CC) and circuit common. The junction of the two resistors R187, R188 produces a reset threshold voltage V_(RST) (e.g., approximately 4.8 V), which is provided to an inverting input of the comparator U181. An output of the comparator U181 is coupled to the supply voltage V_(CC) via a resistor R189 (e.g., having a resistance of approximately 10 kΩ).

The reset circuit 180 also comprises a second comparator U191 having a non-inverting input coupled to the threshold voltage V_(RST) and an output coupled to the timing voltage V_(TIM). The output of the comparator U181 is coupled to an inverting input of the second comparator U191 via a capacitor C190 (e.g., having a capacitance of approximately 1000 pF). A resistor R192 (e.g., having a resistance of approximately 68 kΩ) and a diode D193 are coupled between the inverting input of the comparator U191 and circuit common. A FET Q194 is also coupled between the inverting input and circuit common. The gate of the FET Q194 is pulled up towards the supply voltage V_(CC) through a resistor R195 (e.g., having a resistance of approximately 100 kΩ), and is coupled to the latch voltage V_(LATCH), such that the FET may be rendered conductive and non-conductive in response to the one-shot latch circuit 170.

When the timing voltage V_(TIM) starts out at approximately zero volts, the inverting input of the comparator U172 of the latch circuit 170 is less than the latch threshold voltage V_(TH-L) (e.g., approximately 10.5 V) at the non-inverting input and the output is pulled up towards the supply voltage V_(CC) via the resistor R195 and the diode D196 of the reset circuit 180. The magnitude of the timing voltage V_(TIM) continues to increase at the constant rate until the magnitude of timing voltage exceeds the latch threshold voltage V_(TH-L), at which time, the comparator U172 of the latch circuit 170 drives the output low to approximately zero volts. At this time, the magnitude of the timing voltage V_(TIM) is reduced to approximately the forward voltage drop of the diode D174 (e.g., approximately 0.6 V). Accordingly, the fixed amount of time T_(TIM) that the timing voltage V_(TIM) is generated each half-cycle is a function of the constant rate at which the magnitude of the timing voltage V_(TIM) increases with respect to time dV_(TIM)/dt (i.e., approximately 1.4 V/msec) and the magnitude of the latch threshold voltage V_(TH-L) (i.e., approximately 10.5 V), such that the fixed amount of time T_(TIM) is approximately 7.5 msec each half-cycle. After the magnitude of the timing voltage V_(TIM) has exceeded the latch threshold voltage V_(TH-L), the latch threshold voltage V_(TH-L) is reduced to approximately 0.1 V, such that the comparator U172 continues to drive the output low and the magnitude of the timing voltage V_(TIM) is maintained at approximately 0.6 V.

At the beginning of a half-cycle, the magnitude of the rectified voltage V_(RECT) is below a break-over voltage of the zener diode Z182 of the reset circuit 180 (e.g., approximately 30 V) and the voltage at the non-inverting input of the first comparator U181 is approximately zero volts, such that the output of the first comparator is driven low towards circuit common. When the magnitude of the rectified voltage V_(RECT) exceeds approximately the break-over voltage of the zener diode Z182, the capacitor C184 begins to charge until the magnitude of the voltage at the non-inverting input of the first comparator U181 exceeds the reset threshold voltage V_(RST). The output of the first comparator U181 is then driven high towards the supply voltage V_(CC) and the capacitor C190 conducts a pulse of current into the resistor R192, such that the magnitude of the voltage at the inverting input of the second comparator U191 exceeds the reset threshold voltage V_(RST), and the second comparator pulls the timing voltage V_(TIM) down towards circuit common (i.e., the magnitude of the timing voltage is controlled from approximately 0.6 volts to zero volts). The magnitude of the voltage at the inverting input of the comparator U172 of the latch circuit 170 is now less than the latch threshold voltage V_(TH-L) (i.e., approximately 0.1 V), and the comparator stops pulling the timing voltage V_(TIM) down towards circuit common. In addition, the reset circuit 180 only drives the timing voltage V_(TIM) low for a brief period of time (e.g., approximately 68 μsec) before the capacitor C190 fully charges and then stops conducting the pulse of current into the resistor R192. Accordingly, the second comparator U191 then stops pulling the timing voltage V_(TIM) down towards circuit common, thus allowing the timing voltage to once again begin increasing in magnitude with respect to time at the constant rate.

After the reset circuit 180 resets the generation of the timing voltage V_(TIM) after the beginning of each half-cycle, the comparator U172 of the latch circuit 170 stops pulling the timing voltage V_(TIM) down towards circuit common and the magnitude of the latch voltage V_(LATCH) is pulled high towards the supply voltage V_(CC) via the resistor R195 and the diode D196. At this time, the FET Q194 is rendered conductive, thus maintaining the inverting input of the second comparator U191 less than the reset threshold voltage V_(RST). The FET Q194 is rendered non-conductive when the comparator U172 of the one-shot latch circuit 170 pulls the timing voltage V_(TIM) low near the end of the half-cycle. Thus, the FET Q194 is rendered conductive for most of each half-cycle and prevents the reset circuit 180 from resetting the generation of the timing voltage V_(TIM) until after the latch circuit 170 ceases the generation of the timing voltage, thereby greatly improving the noise immunity of the dimmer switch 100 with respect to impulse noise on the AC line voltage V_(AC).

When the magnitude of the voltage at the non-inverting input of the first comparator U181 of the reset circuit 170 exceeds the reset threshold voltage V_(RST), the output is then driven high towards the supply voltage V_(CC) and the capacitor C190 charges. The FET Q194 is then rendered conductive, and the capacitor C190 remains charged. When the magnitude of the rectified voltage V_(RECT) drops below the break-over voltage of the zener diode Z182 at the end of each half-cycle and the magnitude of the voltage at the non-inverting input of the first comparator U181 drops below the reset threshold voltage V_(RST), the capacitor C190 discharges through the diode D193 and the output of the first comparator U181. However, the magnitude of the voltage at the inverting input of the second comparator U191 remains less than the reset threshold voltage V_(RST), and thus the reset circuit 180 does not reset the generation of the timing voltage V_(TIM) until the magnitude of the voltage at the non-inverting input of the first comparator U181 of the reset circuit 170 rises above the reset threshold voltage V_(RST) at the beginning of the next half-cycle.

Accordingly, the control circuit 115 of the dimmer switch 100 of the first embodiment of the present invention conducts a control current through the LED driver 102 and provides constant gate drive to the bidirectional semiconductor switch 110 after the bidirectional semiconductor switch is rendered conductive. The control circuit 115 is operable to derive zero-crossing timing information from the voltage developed across the LED driver 102, and thus from the control current I_(CNTL) conducted through the LED driver 102. The average magnitude of the control current I_(CNTL) conducted through the LED driver 102 is approximately equal to the sum of the average magnitude of the timing current I_(TIM) and the drive current I_(DR), as well as the other currents drawn by the timing circuit 130 and the trigger circuit 140. The control circuit 115 is operable to render the bidirectional semiconductor switch 110 conductive each half-cycle in response to the variable threshold that is representative of the desired intensity of the LED light source 104 and to maintain the bidirectional semiconductor switch conductive until approximately the end of the present half-cycle. As a result, the conduction time T_(CON) of the drive voltage V_(DR) generated by the trigger circuit 140 has a length that is not dependent upon the length of the fixed amount of time T_(TIM) that the timing circuit 130 generates the timing signal V_(TIM).

FIG. 6 is a simplified schematic diagram of a dimmer switch 200 according to a second embodiment of the present invention. The bidirectional semiconductor switch of the dimmer switch 200 of the second embodiment is implemented as two individual switching transistors, e.g., FETs Q210A, Q210B, coupled in anti-series connection between the hot terminal H and the dimmed hot terminal DH for control of the amount of power delivered to the LED driver 102. The sources of the FETs Q210A, Q210B are coupled together at circuit common. The FETs Q210A, Q210B may comprise metal-oxide semiconductor FETs (MOSFETs) or may alternatively be replaced by any suitable semiconductor switch, such as, for example, insulated gate bipolar junction transistors (IGBT). The FETs Q210A, Q210B have control inputs (i.e., gates) that are coupled to a gate coupling circuit 250, that comprises respective gate resistors R252, R254 (e.g., each having a resistance of approximately 47Ω) for coupling a drive voltage V_(DR-INV) to the gates of the FETs. The drive voltage V_(DR-INV) is the inverse of the drive voltage V_(DR) of the first embodiment as shown in FIGS. 3A and 3B. The FETs Q210A, Q210B are simultaneously controlled to be conductive and non-conductive using the forward phase-control technique, and are operable to be rendered conductive and to remain conductive independent of the magnitude of the load current I_(LOAD) conducted through the FETs.

The dimmer switch 200 comprises a full-wave rectifier bridge that includes the body diodes of the two FETs Q210A, Q210B in addition to two diodes D214A, D214B. The timing circuit 130 of the dimmer switch 200 of the second embodiment operates in the same manner as in the first embodiment. The dimmer switch 200 comprises a variable-threshold trigger circuit 240 that is similar to the variable-threshold trigger circuit 140 of the first embodiment. However, the trigger circuit 240 of the second embodiment comprises a comparator U242 having a non-inverting input that receives the timing voltage V_(TIM) and an inverting input that receives a variable threshold voltage V_(TH) from a potentiometer R244. The trigger circuit 240 operates to drive the drive voltage V_(DR-INV) high towards the supply voltage V_(CC) to render the FETs Q210A, Q210B conductive, and low towards circuit common to render the FETs non-conductive.

FIG. 7 is a simplified block diagram of a reverse-phase control dimmer switch 300 according to a third embodiment of the present invention. As shown in FIG. 7, the bidirectional semiconductor switch is implemented as two FETs Q210A, Q210B coupled in anti-series connection (as in the second embodiment). The dimmer switch 100 comprises an analog control circuit including a voltage reference circuit 320, a timing circuit 330, and a gate drive circuit 340. The voltage reference circuit 320 includes a pass-transistor circuit 360 and a snap-on circuit 370, and operates to generate a reference voltage V_(REF) (e.g., approximately 14.4 volts) from the rectified voltage V_(RECT). The timing circuit 330 receives the reference voltage V_(REF) and generates a timing voltage V_(TIM), which is representative of the target intensity L_(TRGT) of the LED light source 104. The gate drive circuit 340 generates a gate voltage V_(G), which is coupled to the gates of the FETs Q210A, Q210B via the gate coupling circuit 250 for simultaneously rendering the FETs conductive and non-conductive. According to the third embodiment of the present invention, the phase-control voltage V_(PC) generated by the dimmer switch 300 comprises a reverse phase-control voltage. Accordingly, the gate drive circuit 340 operates to render the FETs Q210A, Q210B conductive at the beginning of each half-cycle, and non-conductive at some time during each half-cycle in response to the timing voltage V_(TIM).

FIG. 8 is a simplified timing diagram showing examples of the phase-control voltage V_(PC) generated by the dimmer switch 300, the timing voltage V_(TIM), and the gate voltage V_(G) for driving the FETs Q210A, Q210B according to the third embodiment of the present invention. The phase-control voltage V_(PC) has a magnitude equal to approximately the magnitude of the AC line voltage V_(AC) of the AC power source 105 at the beginning of each half-cycle during a conduction time T_(CON), and has a magnitude of approximately zero volts during the rest of the half-cycle, i.e., during a non-conduction time T_(NC). To generate the phase-control voltage V_(PC), the gate drive circuit 340 drives the gate voltage V_(G) high towards the reference voltage V_(REF) at the beginning of each half-cycle, such that the FETs Q210A, Q210B are rendered conductive (as shown at time t₁ in FIG. 8). At this time, the timing circuit 330 begins generating the timing voltage V_(TIM), which comprises a ramp voltage that increases in magnitude with respect to time at a rate representative of the target intensity L_(TRGT) of the LED light source 104 (i.e., in response to the intensity adjustment actuator 118). When the magnitude of the timing voltage V_(TIM) reaches a maximum timing voltage threshold V_(T-MAX) (e.g., approximately 7.5 volts), the gate drive circuit 340 renders the FETs Q210A, Q210B non-conductive (as shown at time t₂ in FIG. 8). The rate of the timing voltage V_(TIM) is inversely proportional to the target intensity L_(TRGT), i.e., the rate of the timing voltage V_(TIM) increases as the target intensity L_(TRGT) decreases, and decreases as the target intensity L_(TRGT) increases. After the FETs Q210A, Q210B are rendered non-conductive, the gate drive circuit 340 will render the FETs conductive once again at the beginning of the next half-cycle (as shown at time t₃ in FIG. 8).

FIG. 9 is a simplified schematic diagram of the dimmer switch 300 according to the third embodiment of the present invention. As shown in FIG. 9, the pass-transistor circuit 360 comprises an NPN bipolar junction transistor Q362 having a collector coupled to receive the rectifier voltage V_(RECT) through a resistor R364 (e.g., having a resistance of approximately 180 Ω). The base of the transistor Q362 is coupled to the rectifier voltage V_(RECT) through a resistor R365 (e.g., having a resistance of approximately 470 kΩ), and to circuit common through a zener diode Z366 (e.g., having a break-over voltage of approximately 15 volts). The pass-transistor circuit 360 further comprises a storage capacitor C368, which is able to charge through the transistor Q362 and a diode D369 to a voltage equal to approximately the break-over voltage of the zener diode Z366 minus the base-emitter drop of the transistor Q362 and the forward drop of the diode D369. The storage capacitor C368 has, for example, a capacitance of approximately 22 μF, and operates to maintain the reference voltage V_(REF) at an appropriate magnitude (e.g., at least approximately 12 volts) to control the FETs Q210A, Q210B to be conductive (i.e., when there is approximately zero volts generated across the dimmer switch 100) as will be described in greater detail below.

The snap-on circuit 370 is coupled to the storage capacitor Q368 and comprises a PNP bipolar junction transistor Q372. The base of the transistor Q372 is coupled to circuit common through the series combination of a resistor R374 (e.g., having a resistance of approximately 22 kΩ) and a zener diode Z376 (e.g., having a break-over voltage of approximately 12 volts). The reference voltage V_(REF) is generated across a capacitor C378, which is coupled between the collector of the transistor Q372 and circuit common and has, for example, a capacitance of approximately 0.1 μF. The snap-on circuit 370 operates such that the reference voltage V_(REF) is only provided across the capacitor C378 when the magnitude of the voltage across the storage capacitor C368 of the pass-transistor circuit 360 exceeds the break-over voltage of the zener diode Z376 plus the emitter-base drop of the transistor Q372.

The timing circuit 330 receives the reference voltage V_(REF) and generates the timing voltage V_(TIM) across a timing capacitor C332 (e.g., having a capacitance of approximately 10 nF).

The timing circuit 330 includes a constant current source circuit for charging the capacitor C332 at a constant rate to generate the timing voltage V_(TIM). The constant current source circuit comprises a PNP bipolar junction transistor Q334 having an emitter coupled to the reference voltage V_(REF) via a resistor R335 (e.g. having a resistance of approximately 180 kΩ). A voltage divider circuit comprising a potentiometer R336 and two resistors R338, R339 is coupled between the reference voltage V_(REF) and circuit common. For example, the potentiometer R336 may have a resistance ranging from approximately 0 to 500 kΩ, while the resistors R338, R339 may have resistances of approximately 100 kΩ and 82 kΩ, respectively. The junction of the potentiometer R336 and the resistor R338 is coupled to the base of the transistor Q334. The resistance of the potentiometer R336 varies in response to the intensity adjustment actuator 118 of the dimmer switch 100, such that the magnitude of the voltage at the base of the transistor Q334 is representative of the target intensity L_(TRGT). When the potentiometer R336 is not presently being adjusted (i.e., is in a steady state condition), a constant voltage is generated across the resistor R335 and the emitter-base junction of the transistor Q334, such that the transistor Q334 conducts a constant current (having a magnitude dependent upon the magnitude of the voltage at the base of the transistor Q334). Accordingly, the capacitor C332 charges at a rate dependent upon the target intensity L_(TRGT) thus generating the timing voltage V_(TIM) (as shown in FIG. 8).

The gate drive circuit 340 renders the FETs Q210A, Q210B conductive at the beginning of each half-cycle, and non-conductive at some time during each half-cycle in response to the timing voltage V_(TIM) from the timing circuit 330. The gate drive circuit 340 comprises an NPN bipolar junction transistor Q341 and a resistor R342, which is coupled between the collector and base of the transistor Q341 and has a resistance of, for example, approximately 270 kΩ. A diode D343 is coupled between the emitter and the base of the transistor Q341. At the beginning of each half-cycle, the resistor R342 conducts current into the base of the transistor Q341. The transistor Q341 is thus rendered conductive and the reference voltage V_(REF) is coupled to the gates of the FETs Q210A, Q210B via the respective gate resistors R252, R254 to thus render the FETs conductive. As previously mentioned, the storage capacitor C368 of the voltage reference circuit 320 maintains the reference voltage V_(REF) at an appropriate magnitude (i.e., at least approximately 14.4 volts) to maintain the FETs Q210A, Q210B conductive and the voltage developed across the dimmer switch 300 is approximately zero volts.

The timing voltage V_(TIM) is coupled to the base of an NPN bipolar junction transistor Q344 through a zener diode Z345 (e.g., having a break-over voltage of approximately 6.8 volts). When the magnitude of the timing voltage V_(TIM) exceeds approximately the break-over voltage of the zener diode Z345 plus the base-emitter drop of the transistor Q344 (i.e., the maximum timing voltage threshold V_(T-MAX)), the transistor Q344 is rendered conductive. Accordingly, the gate voltage V_(G) is pulled down towards circuit common through the diode D343 thus rendering the FETs Q210A, Q210B non-conductive.

The gate drive circuit 340 also comprises an NPN bipolar junction transistor Q346 coupled across the zener diode Z345. The base of the transistor Q346 is coupled to the junction of two series-connected resistors R347, R348 (e.g., having resistances of approximately 200 kΩ and 10 kΩ respectively). The resistors R347, R348 form a voltage divider coupled between the rectified voltage V_(RECT) and circuit common. The base of the transistor Q346 is also coupled to circuit common via a capacitor C349 (e.g., having a capacitance of approximately 10 nF). When the FETs Q210A, Q210B are rendered non-conductive (in response to the timing voltage V_(TIM) exceeding the maximum timing voltage threshold V_(T-MAX)), the voltage developed across the dimmer switch 300 increases to approximately the magnitude of the AC line voltage V_(AC) of the AC power source 105. As a result, the voltage at the base of the transistor Q346 increases such that the transistor is rendered conductive. Accordingly, the magnitude of the timing voltage V_(TIM) is controlled to approximately zero volts and the transistor Q344 is maintained conductive (thus keeping the FETs Q210A, Q210B non-conductive) until the end of the present half-cycle.

Near the end of the half-cycle, the magnitude of the AC line voltage V_(AC) of the AC power source 105 as well as the magnitude of voltage at the base of the transistor Q346 decrease such that the transistor Q346 is rendered non-conductive. Accordingly, the transistor Q344 is rendered non-conductive and the reference voltage V_(REF) is coupled to the gates of the FETs Q210A, Q210B through the transistor Q341 and the respective gate resistors R252, R254, thus rendering the FETs conductive. In addition, when the transistor Q346 is non-conductive, the timing voltage V_(TIM) of the timing circuit 330 may once again begin increasing in magnitude with respect to time at the rate dependent upon the target intensity L_(TRGT) (as shown in FIG. 8).

FIG. 10 is a simplified schematic diagram of a dimmer switch 400 according to a fourth embodiment of the present invention. The dimmer switch 400 of the fourth embodiment is very similar to the dimmer switch 300 of the third embodiment. However, the dimmer switch 400 of the fourth embodiment comprises a voltage compensation circuit 480, which receives the rectified voltage V_(RECT) and adjusts the timing voltage V_(TIM) to account for changes and fluctuations in the AC line voltage V_(AC) of the AC power source 105 to avoid flickering of the intensity of the LED light source 104. The voltage compensation circuit 480 comprises two resistors R482, R484, which are coupled in series between the rectified voltage V_(RECT) and circuit common, and have, for example, resistances of approximately 1 MΩ and 98 kΩ, respectively. A capacitor C486 is coupled between the junction of the resistors R482, R484 and circuit common, and has, for example, a capacitance of approximately 0.22 μF. The capacitor C486 is coupled to the timing voltage V_(TIM) through a resistor R488 (e.g., having a resistance of approximately 560 kΩ).

The voltage produced across the capacitor C486 is proportional to the magnitude of the AC line voltage V_(AC) of the AC power source 105 when the FETs Q210A, Q210B are non-conductive and the timing voltage V_(TIM) is increasing in magnitude with respect to time. When there are no changes or fluctuations in the magnitude of the AC line voltage V_(AC) of the AC power source 105, the capacitor C486 charges to a steady-state voltage. However, if the magnitude of the AC line voltage V_(AC) changes while the FETs Q210A, Q210B are non-conductive during a half-cycle (e.g., between times t₂ and t₃ in FIG. 8), the magnitude of the voltage across the capacitor C486 will also change, thus resulting in a change in the timing voltage V_(TIM) when the FETs are conductive during the next half-cycle (e.g., between times t₃ and t₄). For example, if the magnitude of the AC line voltage V_(AC) (and thus the magnitude of the voltage across the capacitor C486) increases while the FETs Q210A, Q210B are non-conductive during a half-cycle, the magnitude of the timing voltage V_(TIM) will be greater while the FETs are conductive during the next half-cycle, thus causing the FETs to be rendered non-conductive earlier in the next half-cycle.

FIG. 11 is a simplified schematic diagram of a dimmer switch 500 according to a fifth embodiment of the present invention. The dimmer switch 500 comprises a mechanical air-gap switch S514 and two FETs Q510A, Q510B coupled in anti-series connection between the hot terminal H and the dimmed hot terminal DH for generating the phase-control voltage V_(PC). The dimmer switch 500 comprises an analog control circuit (e.g., a timing circuit 520) for generating a timing voltage V_(TIM) representative of the target intensity L_(TRGT) of the LED light source 104, and a gate drive circuit 530 for rendering the FETs Q510A, Q510B conductive and non-conductive in response to the timing voltage V_(TIM) to thus generate the phase-control voltage V_(PC). According to the fifth embodiment of the present invention, the gate drive circuit 530 is operable to generate two gate voltages V_(G1), V_(G2) for independently controlling the respective FETs Q510A, Q510B on a complementary basis. The FETs Q510A, Q510B are rendered conductive when the magnitudes of the respective gate voltages V_(G1), V_(G2) are controlled to a nominal gate voltage V_(N) (e.g., approximately 9 V) and are rendered non-conductive when the magnitudes of the respective gate voltages V_(G1), V_(G2) are controlled to approximately zero volts. The dimmer switch 500 further comprises an overcurrent protection circuit 540 for rendering the FETs Q510A, Q510B non-conductive in the event of an overcurrent condition in the FETs.

FIG. 12 is a simplified timing diagram showing examples of the phase-control voltage V_(PC) generated by the dimmer switch 500 and the gate voltages V_(G1), V_(G2) for driving the FETs Q510A, Q510B, respectively. According to the fifth embodiment of the present invention, the phase-control voltage V_(PC) comprises a forward phase-control voltage. During the positive half-cycles, the first FET Q510A is rendered conductive and the second FET Q510B is rendered non-conductive when the first gate voltage V_(G1) increases from approximately zero volts to the nominal gate voltage V_(N) (as shown at time t₁), and the second gate voltage V_(G2) decreases from the nominal gate voltage V_(N) to approximately zero volts. At this time, the dimmer switch 500 conducts the load current I_(LOAD) to the LED driver 102 through the first FET Q510A and the body diode of the second FET Q510B. At the beginning of the negative half-cycles, the first FET Q510 remains conductive. However, since the second FET Q510B is non-conductive and the body diode of the second FET Q510B is reversed-biased, the dimmer switch 500 does not conduct the load current I_(LOAD) at this time.

During the negative half-cycles, the first FET Q510A is rendered non-conductive and the second FET Q510B is rendered conductive when the first gate voltage V_(G1) decreases from the nominal gate voltage V_(N) to approximately zero volts and the second gate voltage V_(G2) increases from approximately zero volts to the nominal gate voltage V_(N) (as shown at time t₂). At this time, the dimmer switch 500 conducts the load current I_(LOAD) to the LED driver 102 through the second FET Q510B and the body diode of the first FET Q510A. At the beginning of the positive half-cycles, the second FET Q510B remains conductive, the first FET Q510A remains non-conductive, and the body diode of the first FET Q510A is reversed-biased at this time, such that the dimmer switch 500 does not conduct the load current I_(LOAD) until the first FET Q510A is rendered conductive.

The timing circuit 520 is coupled in series between the hot terminal H and the dimmed hot terminal DH and conducts a timing current I_(TIM) (i.e., a control current) through the LED driver 102 in order to generate the timing voltage V_(TIM) across a capacitor C522 (e.g., having a capacitance of approximately 0.1 μF). The capacitor C522 is operable to charge from the AC power source 105 through resistors R524, R525 (e.g., having resistances of approximately 27 kΩ and 10 kΩ respectively) and a potentiometer R526. The resistance of the potentiometer R526 may range from, for example, approximately 0 kΩ to 300 kΩ and may be controlled by a user of the dimmer switch 500 (e.g., by actuating the slider control) to adjust the target intensity L_(TRGT) of the LED light source 104. A calibration resistor R527 is coupled to potentiometer R526 for calibrating the range of the potentiometer, and has a resistance of, for example, approximately 300 kΩ. Since the capacitor C522 charges through the potentiometer R526, the rate at which the capacitor C522 charges and thus the magnitude of the timing voltage V_(TIM) are representative of the target intensity L_(TRGT) of the LED light source 104.

The drive circuit 530 comprises a diac 532 (e.g., having a break-over voltage V_(BR) of approximately 32 volts) and two pulse transformers 534A, 534B. The diac 532 is coupled in series with the primary windings of the two pulse transformers 534A, 534B. The secondary windings of the pulse transformers 534A, 534B are coupled to respective capacitors C535A, C535B via respective zener diodes Z536A, Z536B (which each have a break-over voltage approximately equal to the nominal gate voltage V_(N), i.e., approximately 9 V). The capacitors C535A, C535B are coupled to the gates of the FETs Q510A, Q510B via gate resistors R538A, R538B, respectively (e.g., having resistances of approximately 47 kΩ). The gate resistors R538A, R538B may alternatively have different resistances in order to change the duration of the switching times of the FETs Q510A, Q510B as is well known in the art.

When the magnitude of the timing voltage V_(TIM) exceeds approximately the break-over voltage V_(BR) of the diac 532, the diac conducts a pulse of current (i.e., a firing current I_(FIRE) as shown in FIG. 12) through the primary windings of the pulse transformers 534A, 534B causing secondary voltages V_(SEC) (e.g., approximately 9V) to be generated across the secondary windings of the pulse transformers. During the positive half-cycles, the capacitor C535A charges from the secondary winding of the first pulse transformer 534A through the zener diode Z536A to approximately the nominal gate voltage V_(N) (i.e., approximately 9 volts). Accordingly, the first gate voltage V_(G1) is driven high from approximately zero volts to the nominal gate voltage V_(N) rendering the first FET Q510A conductive (as shown at time t₁ in FIG. 12). At the beginning of the negative half-cycles, the first FET Q510A is conductive, while the second FET Q510B is non-conductive. Since the body diode of the second FET Q510B is reversed biased at this time, the dimmer switch 500 does not conduct the load current I_(LOAD).

During the negative half-cycles, the firing current I_(FIRE) has a negative magnitude, thus causing the secondary voltages V_(SEC) across the secondary windings of the pulse transformers 534A, 534B to also have negative magnitudes. Accordingly, the zener diode Z536A is reverse-biased during the negative half-cycles, causing the capacitor C535A to discharge through the zener diode Z536A, such that the voltage across the capacitor C535A is driven to approximately zero volts. As a result, the first gate voltage V_(G1) is driven low from the nominal gate voltage V_(N) to approximately zero volts rendering the first FET Q510A non-conductive (as shown at time t₂ in FIG. 12). In addition, the zener diode Z536B coupled to the secondary winding of the second pulse transformer 534B is forward-biased in the negative half-cycles, such that the capacitor C535B charges to approximately the nominal gate voltage V_(N) and the second FET Q510B is rendered conductive during the negative half-cycles (as shown at time t₂ in FIG. 12). Accordingly, the FETs Q510A, Q510B are driven in a complementary manner, such that—at all times—at least one FET is conductive, while the other FET is non-conductive. As a result, the FETs Q510A, Q510B are driven to be conductive for approximately the period T_(HC) of a half-cycle and non-conductive for the period T_(HC) of a half-cycle.

The timing circuit 520 also comprises a diac 528 (e.g., having a break-over voltage of approximately 64V) coupled to the potentiometer R526. The diac 528 provides voltage compensation by adjusting the voltage provided to the potentiometer R526 to compensate for variations in the AC line voltage V_(AC) provided by the AC power source 105. The diac 528 has a negative impedance transfer function, such that the voltage across the diac increases as the current through the diac decreases. Thus, as the voltage across the dimmer switch 500 (i.e., between the hot terminal H and the dimmed hot terminal DH) decreases, the current through the resistor R524 and the diac 528 decreases. As a result, the voltage across the diac 528 increases, thus causing the current flowing through the potentiometer R526 to increase and the firing capacitor C522 to charge at a faster rate. This results in an increased conduction time T_(CON) of the FETs Q510A, Q510B during the present half-cycle to compensate for the decreased voltage across the dimmer switch 500, thereby maintaining the intensity of the LED light source 104 constant.

The drive circuit 530 is characterized as having inherent shorted-FET protection. In the event that one of the FETs Q510A, Q510B fails shorted, the drive circuit 530 is operable to drive the other, non-shorted FET into full conduction, such that the load current I_(LOAD) is not asymmetric. Asymmetric current can cause some types of lighting loads to overheat. For example, if the second FET Q510B fails shorted, the full AC waveform will be provided to the LED driver 102 during the negative half-cycles. Since there will be approximately zero volts produced across the dimmer switch 500 during the negative half-cycles when second FET Q510B is shorted, the capacitor C522 of the timing circuit 520 will not charge, the diac 532 of the drive circuit 330 will not conduct the pulse of the firing current I_(FIRE), and the voltage across the capacitor C535A will not be driven to zero volts to render the first FET Q510A non-conductive during the negative half-cycles. Accordingly, the first FET Q510A will remain conductive during both half-cycles and the load current I_(LOAD) will be substantially symmetric. The second FET Q510B is controlled to be conductive in a similar manner if the first FET Q510A has failed shorted.

The overcurrent protection circuit 540 comprises a sense resistor R542 (e.g., having a resistance of approximately 0.015Ω). The sense resistor R542 is coupled between the sources of the FETs Q510A, Q510B, such that a voltage representative of the magnitude of the load current I_(LOAD) is generated across the sense resistor. The voltage generated across the sense resistor R542 is provided to the base of a first NPN bipolar junction transistor (BJT) Q544. The first transistor Q544 is coupled across the capacitor C535A and operates to protect the first FET Q510A in the event of an overcurrent condition during the positive half-cycles. When the magnitude of the load current I_(LOAD) exceeds a predetermined current limit (e.g., approximately 46.6 amps) such that the voltage generated across the sense resistor R542 exceeds the rated base-emitter voltage (e.g., approximately 0.7 volts) of the first transistor Q544, the first transistor is rendered conductive. Accordingly, the first transistor Q544 pulls the first gate voltage V_(G1) at the gate of the first FET Q510A down towards zero volts, thus rendering the first FET non-conductive. The overcurrent protection circuit 540 further comprises a second NPN bipolar junction transistor Q546, which is coupled across the capacitor C535B and operates to protect the second FET Q510B during the negative half-cycles. When the magnitude of the load current I_(LOAD) exceeds the predetermined current limit, the second transistor Q546 is rendered conductive, thus pulling the second gate voltage V_(G2) at the gate of the second FET Q510B down towards zero volts and rendering the second FET non-conductive.

FIG. 13 is a simplified schematic diagram of a dimmer switch 600 according to a sixth embodiment of the present invention. The dimmer switch 600 comprises a drive limit circuit 650, which is coupled in series with the diac 532 and the primary windings of the two pulse transformers 534A, 534B of the drive circuit 530. The drive limit circuit 650 operates to limit the number of times that the drive circuit 530 attempts to render the FETs Q510A, Q510B conductive during a specific half-cycle. For example, if the overcurrent protection circuit 540 renders one of the FETs Q510A, Q510B non-conductive, the drive limit circuit 650 prevents the drive circuit 530 from attempting to render the respective FET conductive again during the present half-cycle.

When the diac 532 fires each half-cycle, the drive limit circuit 650 conducts the firing current I_(FIRE) and generates an offset voltage V_(OFFSET) across a capacitor C652A during the positive half-cycles and a capacitor C652B during the negative half-cycles. The capacitor C452A charges through a diode D654A during the positive half-cycles, and the capacitor C452B charges through a diode D654B during the negative half-cycles. For example, the capacitors C652A, C652B may have capacitances of approximately 0.1 μF. Discharge resistors R656A, R656B are coupled in parallel with the capacitors C652A, C652B, respectively, and each have a resistance of, for example, approximately 33 kΩ. The drive limit circuit 450 further comprises two zener diodes Z658A, Z658B coupled in anti-series connection and each having the same break-over voltage V_(Z) (e.g., approximately 40V). The zener diodes Z658A, Z658B are coupled to the timing circuit 520 to limit the magnitude of the timing voltage V_(TIM) to a clamp voltage V_(CLAMP), i.e., approximately the break-over voltage V_(Z), in both half-cycles.

At the beginning of a positive half-cycle, the capacitor C652A of the drive limit circuit 540 has no charge, and thus, no voltage is developed across the capacitor. The timing voltage signal V_(TIM) increases until the magnitude of the timing voltage V_(TIM) exceeds approximately the break-over voltage V_(BR) of the diac 532. When the diac 532 fires, the diode D654A and the capacitor C652A conduct pulse of the firing current I_(FIRE) and the offset voltage V_(OFFSET) (e.g., approximately 12 volts) is developed across the capacitor C652A. After the diac 532 has finished conducting the firing current I_(FIRE), the voltage across the capacitor C522 decreases by approximately a break-back voltage (e.g., approximately 10 volts) of the diac 532 to a predetermined voltage V_(P) (e.g., approximately 22 volts). If the overcurrent protection circuit 540 renders one of the FETs Q510A, Q510B non-conductive, the timing voltage signal V_(TIM) will begin to increase again. The magnitude of the timing voltage V_(TIM) must exceed approximately the break-over voltage V_(BR) of the diac 532 plus the offset voltage V_(OFFSET) across the capacitor C652A (i.e., approximately 44 volts) in order for the diac 532 to conduct the pulse of the firing current I_(FIRE) once again. However, because the zener diode Z658A limits the timing voltage V_(TIM) to the break-over voltage V_(Z) (i.e., approximately 40 volts), the timing voltage V_(TIM) is prevented from exceeding the voltage threshold V_(TH). Accordingly, the drive circuit 530 is prevented from repeatedly attempting to render the FETs Q510A, Q510B conductive during each half-cycle in the event of an overcurrent condition.

The timing voltage V_(TIM) is prevented from exceeding the voltage threshold V_(TH) until the voltage ΔV across the capacitor C652A decays to approximately the break-over voltage V_(Z) of the zener diode Z658A minus the break-over voltage V_(BR) of the diac 532. The capacitor C652A discharges slowly through the discharge resistor R656A, such that the time required for the voltage ΔV across the capacitor C652A to decay to approximately the break-over voltage V_(Z) of the zener diode Z658A minus the break-over voltage V_(BR) of the diac 532 is long enough such that the drive circuit 530 only attempts to render the FETs Q510A, Q510B conductive once during each half-cycle. The voltage across the capacitor C652A decays to substantially zero volts during the negative half-cycle such that the voltage across the capacitor C652A is substantially zero volts at the beginning of the next positive half-cycle. The capacitor C652B, the diode D654B, the discharge resistor R656B, and the zener diode Z658B of the drive limit circuit 650 operate in a similar fashion during the negative half-cycles. An example of the drive limit circuit 650 is described in greater detail in commonly-assigned U.S. Pat. No. 7,570,031, issued Aug. 4, 2009, entitled METHOD AND APPARATUS FOR PREVENTING MULTIPLE ATTEMPTED FIRINGS OF A SEMICONDUCTOR SWITCH IN A LOAD CONTROL DEVICE, the entire disclosure of which is hereby incorporated by reference.

FIG. 14 is a simplified schematic diagram of a dimmer switch 700 according to a seventh embodiment of the present invention. The dimmer switch 700 comprises a drive circuit 730 that includes a single pulse transformer 734. The pulse transformer 734 has a single primary winding and secondary winding having a tap connection 734′. The diac 532 is coupled in series with the single primary winding of the pulse transformer 734. The series combination of the zener diode Z536A and the capacitor C535A is coupled between one end of the secondary winding and the tap connection 734′ of the pulse transformer 734. The series combination of the diode Z536B and the capacitor C535B is coupled between the other end of the secondary winding and the tap connection 734′ of the pulse transformer 734. The drive circuit 730 of the seventh embodiment operates to render the FETs Q510A, Q510B conductive and non-conductive in the same manner as the drive circuit 530 of the fifth embodiment.

FIG. 15 is a simplified schematic diagram of a dimmer switch 800 according to an eighth embodiment of the present invention. The dimmer switch 800 comprises a mechanical air-gap switch S814 and two FETs Q810A, Q810B coupled in anti-series connection between the hot terminal H and the dimmed hot terminal DH for control of the amount of power delivered to the connected LED driver 102. As in the fifth, sixth, and seventh embodiments, the FETs Q810A, Q810B have control inputs (i.e., gates) that receive respective gate voltages V_(G1), V_(G2) for rendering the FETs conductive and non-conductive. The LED light source 104 is off when the switch S814 is open, and is on when the switch is closed. The dimmer switch 800 comprises a control circuit that includes a timing circuit 820 and a power supply 880 and is operable to conduct a control current I_(CNTL) through the LED driver 102. The timing circuit 820 conducts a timing current I_(TIM) in order to generate a timing voltage V_(TIM) (as in the fifth embodiment). The dimmer switch 800 further comprises a drive circuit 830 for rendering the FETs 810A, Q810B conductive and non-conductive in response to the timing voltage V_(TIM) and an overcurrent protection circuit 860 for rendering the FETs 810A, Q810B non-conductive in response to an overcurrent condition through the FETs.

The power supply 880 generates a DC supply voltage V_(S) (e.g., approximately 14.4 volts) for powering the drive circuit 830 and the overcurrent protection circuit 860. The power supply 880 conducts a charging current I_(CHRG) through the LED driver 102 when the dimmer switch 800 is not conducting the load current I_(LOAD) to the LED driver and the magnitude of the voltage developed across the dimmer switch is approximately equal to the magnitude of the AC line voltage V_(AC). The control current I_(CNTL) conducted through the LED driver 102 is approximately equal to the sum of the timing current I_(TIM) of the timing circuit 820 and the charging current I_(CHRG) of the power supply 880.

The power supply 880 comprises a diode D881 coupled to the hot terminal H (via the switch S814), such that the power supply 880 only charges during the positive half-cycles of the AC power source 105. The power supply 880 includes a pass-transistor circuit that operates to generate the supply voltage V_(S) across a capacitor C882 (e.g., having a capacitance of approximately 10 μF). The pass-transistor circuit comprises an NPN bipolar junction transistor Q883, a resistor R884 (e.g., having a resistance of approximately 220Ω), a resistor R885 (e.g., having a resistance of approximately 470 kΩ), and a zener diode Z886. The capacitor C882 is coupled to the emitter of the transistor Q883, such that the capacitor is able to charge through the transistor. The zener diode Z886 is coupled to the base of the transistor Q883 and has a break-over voltage of, for example, approximately 15V, such that the capacitor C882 is able to charge to a voltage equal to approximately the break-over voltage minus the base-emitter drop of the transistor.

The power supply 880 further comprises snap-on circuit including a PNP bipolar junction transistor Q887, a resistor R888 (e.g., having a resistance of approximately 22 kΩ), and a zener diode Z889. The resistor R888 and the zener diode Z889 are coupled in series with the base of the transistor Q887, and the collector of the transistor Q887 is coupled to a capacitor C890. The zener diode Z889 has a break-over voltage of, for example, approximately 12 V, such that the voltage across the capacitor C882 is coupled across the capacitor C890 when the magnitude of the voltage across the capacitor C882 exceeds approximately the break-over voltage of the zener diode Z889 plus the emitter-base drop of the transistor Q887. When the magnitude of the voltage across the capacitor C882 drops below approximately the break-over voltage of the zener diode Z889 plus the emitter-base drop of the transistor Q887, the voltage across the capacitor C882 is disconnected from the capacitor C890, such that the supply voltage V_(S) will drop to approximately circuit common (i.e., approximately zero volts).

The timing circuit 820 conducts the timing current I_(TIM) and generates the timing voltage V_(TIM) across a capacitor C822 (e.g., having a capacitance of approximately 0.047 μF). The capacitor C822 charges from the AC power source 105 through resistors R824, R825 (e.g., having resistances of approximately 27 kΩ and 10 kΩ respectively) and a potentiometer R826 (e.g., having a resistance ranging from approximately 0 kΩ to 300 kΩ). A calibration potentiometer R827 is coupled across the potentiometer R826 and has, for example, a resistance ranging from approximately 0 to 500 kΩ. The timing circuit 820 further comprises a diac 828, which has a break-over voltage of, for example, approximately 64V, and operates to provide voltage compensation for the timing circuit (in a similar manner as the diac 528 of the timing circuit 520 of the fifth embodiment).

The drive circuit 830 generates the gate voltages V_(G1), V_(G2) for rendering the FETs Q810A, Q810B conductive and non-conductive on a complementary basis in response to the timing voltage V_(TIM) of the timing circuit 820. The drive circuit 830 comprises a diac 832 (e.g., having a break-over voltage of approximately 32 volts), a resistor R834 (e.g., having a resistance of approximately 680Ω), and two optocouplers U835A, U835B. When the magnitude of the timing voltage V_(TIM) exceeds approximately the break-over voltage of the diac 832, the diac conducts a firing current I_(FIRE) through the input photodiode of the first optocoupler U835A during the positive half-cycles, and through the input photodiode of the second optocoupler U835B during the negative half-cycles. Accordingly, the output phototransistor of the first optocoupler U835A is rendered conductive during the positive half-cycles, and the output phototransistor of the second optocoupler U835B is rendered conductive during the negative half-cycles. The output phototransistors of the optocouplers U835A, U835B are between the supply voltage V_(S) and circuit common through respective resistors R836, R838, which each have resistances of, for example, approximately 4.7

The output phototransistors of the optocouplers U835A, U835B are also coupled to set-reset (SR) latches U840A, U840B, U840C, U840D, which operate to generate the gate voltages V_(G1), V_(G2) and to thus render the FETs Q810A, Q810B conductive and non-conductive on the complementary basis. For example, the SR latches U840A, U840B, U840C, U840D may be implemented as part of a single integrated circuit (IC), which may be powered by the supply voltage V_(S). As shown in FIG. 15, the output phototransistor of the first optocoupler U835A is coupled to the set input of the first SR latch U840A and to the reset input of the second SR latch U840B. The output phototransistor of the second optocoupler U835B is coupled to the set input of the second SR latch U840B and to the reset input of the first SR latch U840A. The output of the first SR latch U840A is coupled to the gate of the first FET Q810A and the output of the second SR latch U840B is coupled to the gate of the second FET Q810B through respective resistors R842, R852, which each have a resistance of, for example, approximately 47 kΩ.

When the output phototransistor of the first optocoupler U835A is rendered conductive during the positive half-cycles, the output of the first SR latch U840A is driven high towards the supply voltage V_(S) (thus rendering the first FET Q810A conductive), while the output of the second SR latch U840B is driven low towards circuit common (thus rendering the second FET Q810B non-conductive). Similarly, when the output phototransistor of the second optocoupler U835B is rendered conductive during the negative half-cycles, the output of the second SR latch U840B is driven high towards the supply voltage V_(S) (thus rendering the second FET Q810B conductive), while the output of the first SR latch U840A is driven low towards circuit common (thus rendering the first FET Q810A non-conductive). Since the set input of the first SR latch U840A is coupled to the reset input of the second SR latch U840B, and the set input of the second SR latch is coupled to the reset input of the first SR latch, the FETs Q810A, Q810B are driven in a complementary manner (as in the fifth embodiment), such that one of the FETs is conductive, while the other FET is non-conductive.

The overcurrent protection circuit 860 is coupled to the set inputs of the third and fourth SR latches U840C, U840D for rendering the FETs Q810A, Q810B non-conductive in the event of an overcurrent condition through the FETs. The output of the third SR latch U840C is coupled to the base of an NPN bipolar junction transistor Q844 via a resistor R846 (e.g., having a resistance of approximately 18 kΩ). The collector of the transistor Q844 is coupled to the gate of the first FET Q810A via a resistor R848 (e.g., having a resistance of approximately 330Ω). The drive circuit 830 comprises a similar circuit for coupling the output of the fourth SR latch U840D to the gate of the second FET Q810B.

The overcurrent protection circuit 860 comprises a sense resistor R870 (e.g., having a resistance of approximately 0.015Ω). The sense resistor R870 is coupled in series between the FETs Q810A, Q810B, and circuit common is referenced to one side of the sense resistor (as shown in FIG. 10), such that the magnitude of the voltage generated across the sense resistor is proportional to the magnitude of the load current I_(LOAD). The sense resistor R870 is coupled to the base of an NPN bipolar junction transistor Q861 via a resistor R862 (e.g., having a resistance of approximately 2.2 kΩ). A resistor R863 is coupled between the base and the emitter of the transistor Q861 and has a resistance of, for example, approximately 4.7 kΩ The emitter of the transistor Q861 is coupled to circuit common and the collector is coupled to the supply voltage V_(S) via two resistors R864, R865 (e.g., having resistances of approximately 18 kΩ and 4.7 kΩ, respectively). The junction of the resistors R864, R865 is coupled to the base of a PNP bipolar junction transistor Q866. The emitter of the transistor Q866 is coupled to the supply voltage V_(S) and the collector is coupled to circuit common through a resistor R867 (e.g., having a resistance of approximately 510Ω). The collector of the transistor Q866 is coupled to the set input of the third SR latch U840C for rendering the first FET Q810A non-conductive in the event of overcurrent conditions during the positive half-cycles. The overcurrent protection circuit 860 comprises a similar circuit (including transistors Q871, Q876, and resistors R872, R873, R874, R875, R877) for rendering the second FET Q810B non-conductive in the event of overcurrent conditions during the negative half-cycles.

In the event of an overcurrent condition during a positive half-cycle, the overcurrent protection circuit 860 drives the set input of the third SR latch U840C high towards the supply voltage V_(S). Thus, the transistor Q844 is rendered conductive pulling the gate voltage V_(G1) down towards circuit common and rendering the first FET Q810A non-conductive. The output phototransistor of the second optocoupler U835B is coupled to the reset input of the third SR latch U840C, such that the overcurrent protection is reset during the next half-cycle (i.e., the negative half-cycle). Specifically, when the output phototransistor of the second optocoupler U835B is rendered conductive during the negative half-cycles, the reset input of the third SR latch U840C latch is driven high towards the supply voltage V_(S), thus rendering the transistor Q844 non-conductive and allowing the first SR latch U840A to control the first FET Q810A. Similarly, the overcurrent protection circuit 860 drives the set input of the fourth SR latch U840D high towards the supply voltage V_(S), thus rendering the second FET Q810B non-conductive in the event of an overcurrent condition during a negative half-cycle. The reset input of the fourth SR latch U840D is driven high when the output phototransistor of the first optocoupler U835A is rendered conductive during the positive half-cycles, thus allowing the second SR latch U840B to once again control the second FET Q810B.

FIG. 16 is a simplified schematic diagram of a dimmer switch 900 according to a ninth embodiment of the present invention. The dimmer switch 900 of the ninth embodiment is similar to the dimmer switch 100 of the first embodiment (as shown in FIG. 4). However, the dimmer switch 900 of the ninth embodiment comprises a digital control circuit 915 having a microprocessor 930 for generating a drive voltage V_(DR) (which is the same as the drive voltage V_(DR) of the first embodiment shown in FIGS. 3A and 3B). Alternatively, the microprocessor 930 may be implemented as a microcontroller, a programmable logic device (PLD), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or any suitable controller or processing device. As shown in FIG. 16, the bidirectional semiconductor switch 110 is implemented as the triac 110′. Alternatively, the bidirectional semiconductor switch 110 of the dimmer switch 900 could be implemented as two FETs in anti-series connection that are simultaneously controlled to be conductive and non-conductive (i.e., in a similar manner as the FETs Q210A, Q210B of the dimmer switch 200 of the second embodiment).

The digital control circuit 915 also comprises a power supply 920 operable to conduct a charging current I_(CHRG) through the LED driver 102 in order to generate a DC supply voltage V_(CC). For example, the power supply 920 may comprise a pass-transistor circuit (as in the dimmer switch 100 of the first embodiment shown in FIG. 4) or any suitable power supply that does not draw a large charging current through the LED driver 102. The digital control circuit 915 comprises a voltage divider having two resistors R934, R935 for generating a scaled voltage V_(SCALED) having a magnitude suitable to be provided to the microprocessor 930. The scaled voltage V_(SCALED) is representative of the voltage developed across the bidirectional semiconductor switch 110. The microprocessor 930 may have an analog-to-digital converter (ADC) for sampling the scaled voltage V_(SCALED), such that the microprocessor 930 is operable to determine the zero-crossings of the phase control voltage V_(PC) in response to the voltage developed across the bidirectional semiconductor switch 110.

The digital control circuit 915 further comprises a toggle tactile switch S_(TOGGLE), a raise tactile switch S_(RAISE), and a lower tactile switch S_(LOWER) for receiving user inputs. The toggle tactile switch S_(TOGGLE) may be mechanically coupled to a toggle actuator or push button. The raise and lower switches S_(RAISE), S_(LOWER) may be mechanically coupled to, for example, separate raise and lower buttons, respectively, or to a rocker switch having an upper portion and a lower portion. The toggle switch S_(TOGGLE) is coupled in series with a resistor R936 between the supply voltage V_(CC) and circuit common, and generates a toggle control signal V_(TOGGLE). The raise switch S_(RAISE) is coupled in series with a resistor R938 between the supply voltage V_(CC) and circuit common, and generates a raise control signal V_(RAISE). The lower switch S_(LOWER) is coupled in series with a resistor R938 between the supply voltage V_(CC) and circuit common, and generates a lower control signal V_(LOWER). The toggle control signal V_(TOGGLE), the raise control signal V_(RAISE), and the lower control signal V_(LOWER) are received by the microprocessor 930. The microprocessor 930 is operable to toggle the LED light source 104 on and off in response to subsequent actuations of the toggle switch S_(TOGGLE). The microprocessor 930 is operable to increase the target intensity L_(TRGT) of the LED light source 104 in response to actuations of the raise switch S_(RAISE) and to decrease the target intensity L_(TRGT) in response to actuations of the lower switch S_(LOWER). Alternatively, the digital control circuit 915 could comprise a potentiometer for generating a DC voltage that is representative of the desired intensity of the LED light source 104 and varies, for example, in magnitude in response to the position of an intensity adjustment actuator of the dimmer switch 900 (i.e., similar to the potentiometer R144 and the intensity adjustment actuator 118 of the dimmer switch 100 of the first embodiment).

In addition, the microprocessor 930 of the dimmer switch 900 may alternatively be operable to receive a digital message from a wired or wireless signal receiver. For example, the digital control circuit 915 of the dimmer switch 900 may comprise a radio-frequency (RF) transceiver (not shown) and an antenna (not shown) for transmitting and receiving RF signals. The microprocessor 930 may be operable to control the bidirectional semiconductor switch 110 in response to the digital messages received via the RF signals. Alternatively, the dimmer switch 900 may simply comprise an RF receiver or an RF transmitter for only receiving or transmitting RF signals, respectively. Examples of RF load control devices and antennas for wall-mounted load control devices are described in greater detail in commonly-assigned U.S. Pat. No. 5,982,103, issued Nov. 9, 1999, and U.S. Pat. No. 7,362,285, issued Apr. 22, 2008, both entitled COMPACT RADIO FREQUENCY TRANSMITTING AND RECEIVING ANTENNA AND CONTROL DEVICE EMPLOYING SAME, the entire disclosures of which are hereby incorporated by reference.

FIG. 17 is a simplified flowchart of a switch procedure 1000 executed by the microprocessor 930 in response to an actuation of one of the raise switch S_(RAISE) or the lower switch S_(LOWER) at step 1010 (i.e., if either of the raise control signal V_(RAISE) and the lower control signal V_(LOWER) are pulled down to circuit common). If the raise switch S_(RAISE) is actuated at step 1012, the microprocessor 930 increases the target intensity L_(TRGT) of the LED light source 104 at step 1014 by decreasing a firing time T_(FIRE) (which is approximately equal to the non-conduction time T_(NC) shown in FIGS. 3A and 3B). If the lower switch S_(LOWER) is actuated at step 1016, the microprocessor 930 decreases the target intensity L_(TRGT) of the LED light source 104 by increasing the firing time T_(FIRE) at step 1018, before the button procedure 1000 exits.

FIG. 18 is a simplified flowchart of a control procedure 1100 periodically executed by the microprocessor 930 (e.g., every 100 μsec) to sample the scaled voltage V_(SCALED) and generate the drive voltage V_(DR). First, the microprocessor 930 samples the scaled voltage V_(SCALED) using the ADC at step 1110. At step 1112, the microprocessor 930 determines if the scaled voltage V_(SCALED) is increasing in magnitude and if the present sample is greater than the previous sample in order to detect a positive-going transition of the scaled voltage V_(SCALED) across a zero-crossing threshold. If the microprocessor 930 detects a positive-going transition across the zero-crossing threshold at step 1112 and a RESET flag is set at step 1114, the microprocessor 930 clears the RESET flag at step 1116. The microprocessor 930 then initializes a timer to zero and starts the timer increasing in value with respect to time at step 1118, before the control procedure 1100 exits. If the RESET flag is not set at step 1114, the microprocessor 930 does not restart the timer at step 1118.

If the timer is equal to the firing time T_(FIRE) at step 1120, the microprocessor 930 drives the drive voltage V_(DR) low to approximately circuit common to render the bidirectional semiconductor switch 110 conductive at step 1122, and the control procedure 1100 exits. If the time is equal to a total time T_(TOTAL) at step 1124, the microprocessor 930 drives the drive voltage V_(DR) high to approximately the supply voltage V_(CC) to render the bidirectional semiconductor switch 110 non-conductive at step 1126. The total time T_(TOTAL) may be equal to the fixed amount of time T_(TIM) that the timing circuit 130 generates the timing voltage V_(TIM) in the dimmer switch 100 of the first embodiment (i.e., approximately 7.5 msec). At step 1128, the microprocessor 930 sets the RESET flag at step 1128, and the control procedure 1100 exits. The RESET flag allows the microprocessor 930 to ensure that the timer is not restarted until after the total time T_(TOTAL).

While the present invention has been described with reference to the high-efficiency lighting load 101 having the LED driver 102 for controlling the intensity of the LED light source 104, the dimmer switches 100, 200, 300, 400, 500, 600, 700, 800, 900 could be used to control the amount of power delivered to other types of lighting loads (such as incandescent lamps, halogen lamps, magnetic low-voltage lamps, electronic low-voltage lamps), other types of electrical loads (such as motor and fan loads), and other types of load regulation devices (such as electronic dimming ballasts for fluorescent lamps).

This application is related to commonly-assigned U.S. patent application Ser. No. 12/953,057 filed Nov. 23, 2010, entitled TWO-WIRE ANALOG FET-BASED DIMMER SWITCH, the entire disclosure of which is hereby incorporated by reference.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims. 

What is claimed is:
 1. A load control device for controlling the amount of power delivered from an AC power source to an electrical load, the load control device comprising: a thyristor adapted to be coupled in series electrical connection between the AC power source and the electrical load for conducting a load current from the AC power source to the electrical load, the thyristor having a gate for rendering the thyristor conductive; and a control circuit generating a gate drive signal for rendering the thyristor conductive at a firing time during each half-cycle of the AC power source, the control circuit comprising a power supply for generating a supply voltage, the power supply adapted to conduct a charging current through the load for generating the supply voltage; wherein the control circuit is operable to maintain the thyristor conductive after the firing time, such that the thyristor is able to remain conductive independent of the magnitude of the load current conducted through the thyristor and to conduct the load current to and from the electrical load during a single half-cycle of the AC power source.
 2. The load control device of claim 1, wherein the supply voltage allows the control circuit to maintain the thyristor conductive after the firing time, so that the thyristor remains conductive independent of the magnitude of the load current conducted through the thyristor.
 3. The load control device of claim 2, wherein the control circuit receives a signal representative of a voltage developed across the thyristor, the control circuit operable to determine a half-cycle start time near the beginning of a half-cycle of the AC power source in response to the signal representative of the voltage developed across the thyristor.
 4. The load control device of claim 3, wherein the control circuit: drives the gate drive signal to a first magnitude to render the thyristor conductive after a first variable amount of time has elapsed since the half-cycle start time; maintains the gate drive signal at the first magnitude after the thyristor is rendered conductive; drives the gate drive signal to a second magnitude after a second fixed amount of time has elapsed since the half-cycle start time to allow the thyristor to commutate off; controls the second fixed amount of time to be approximately equal during each half-cycle of the AC power source; and varies the first variable amount of time in response to a desired amount of power to be delivered to the load to thus control the amount of power delivered to the load to the desired amount.
 5. The load control device of claim 4, wherein the control circuit comprises: a timing circuit for generating a timing signal that increases in magnitude with respect to time, the timing circuit starting to generate the timing signal at the half-cycle start time; and a drive circuit for receiving the timing voltage and rendering the thyristor conductive each half-cycle in response to the magnitude of the timing signal, so as to control the amount of power delivered to the electrical load to a desired amount; wherein the timing circuit is operable to continue generating the timing signal after the thyristor is rendered conductive at the firing time each half-cycle, such that the drive circuit continues to render the thyristor conductive and the thyristor remains conductive independent of the magnitude of the load current conducted through the thyristor.
 6. The load control device of claim 5, wherein the timing signal increases in magnitude with respect to time at a constant rate, and the timing circuit stops generating the timing signal after the second fixed amount of time has elapsed since the half-cycle start time.
 7. The load control device of claim 6, wherein the drive circuit drives the gate drive signal to the first magnitude to render the thyristor conductive when the magnitude of the timing signal exceeds a variable threshold representative of the desired amount of power to be delivered to the load, and drives the gate drive signal to the second magnitude to allow the thyristor to commutate off when the timing circuit stops generating the timing signal.
 8. The load control device of claim 6, wherein the first variable amount of time has a length that is not dependent upon the length of the second fixed amount of time that the timing circuit generates the timing signal.
 9. The load control device of claim 1, wherein the control circuit comprises a microprocessor powered by the supply voltage.
 10. The load control device of claim 9, further comprising: an RF receiver powered by the supply voltage and operable to receive an RF signal; wherein the microprocessor is coupled to the RF receiver for determining a desired amount of power to be delivered to the load in response to the received RF signal.
 11. The load control device of claim 1, wherein the control circuit controls the gate drive signal to allow the control circuit to commutate off before the end of the half-cycle.
 12. The load control device of claim 1, wherein the power supply conducts the charging current through the load when the thyristor is non-conductive to generate the supply voltage.
 13. The load control device of claim 1, wherein the thyristor comprises a triac.
 14. The load control device of claim 1, wherein the electrical load comprises a high-efficiency lighting load and the load control device comprises a dimmer switch.
 15. A load control device for controlling the amount of power delivered from an AC power source to an electrical load to a desired amount of power, the load control device comprising: a bidirectional semiconductor switch adapted to be coupled in series electrical connection between the AC power source and the electrical load for conducting a load current from the AC power source to the electrical load, the bidirectional semiconductor switch having a control input for rendering the bidirectional semiconductor switch conductive and non-conductive; and a digital control circuit receiving a signal representative of a voltage developed across the bidirectional semiconductor switch, the digital control circuit operable to determine a half-cycle start time near the beginning of a half-cycle of the AC power source in response to the signal representative of the voltage developed across the bidirectional semiconductor switch, the digital control circuit comprising a power supply for generating a supply voltage, the power supply adapted to conduct a charging current through the load for generating the supply voltage; wherein the digital control circuit is configured to: generate a gate drive signal that is operatively coupled to the control input of the bidirectional semiconductor switch; drive the gate drive signal to a first magnitude to render the bidirectional semiconductor switch conductive after a first variable amount of time has elapsed since the half-cycle start time; maintain the gate drive signal at the first magnitude after the bidirectional semiconductor switch is rendered conductive, such that the bidirectional semiconductor switch remains conductive independent of the magnitude of the load current conducted through the bidirectional semiconductor switch; drive the gate drive signal to a second magnitude to render the bidirectional semiconductor switch non-conductive after a second fixed amount of time has elapsed since the half-cycle start time; control the second fixed amount of time to be approximately equal during each half-cycle of the AC power source; vary the first variable amount of time in response to the desired amount of power to be delivered to the load to thus control the amount of power delivered to the load to the desired amount.
 16. The load control device of claim 15, wherein the digital control circuit comprises a digital processing device powered by the supply voltage.
 17. The load control device of claim 16, further comprising: an intensity adjustment actuator adapted to be actuated for adjusting the desired amount of power delivered to the load; wherein the digital processing device is operable to determine the desired amount of power to be delivered to the load in response to the intensity adjustment actuator.
 18. The load control device of claim 17, wherein the digital control circuit further comprises raise and lower tactile switches coupled to the digital processing device, and the intensity adjustment actuator comprises a rocker switch for actuating the raise and lower tactile switches for respectively increasing and decreasing the desired amount of power to be delivered to the load.
 19. The load control device of claim 17, wherein the intensity adjustment actuator comprises a potentiometer coupled to the digital processing device, and the intensity adjustment actuator comprises a slider knob mechanically coupled to the potentiometer, such that the potentiometer generates a DC voltage representative of the desired amount of power to be delivered to the load.
 20. The load control device of claim 16, wherein the power supply conducts the charging current through a storage capacitor and the load when the bidirectional semiconductor switch is non-conductive in order to generate the supply voltage across the storage capacitor.
 21. The load control device of claim 16, further comprising: an RF receiver coupled to the digital processing device and operable to receive an RF signal; wherein the digital processing device is operable to determine the desired amount of power to be delivered to the load in response to the RF signal.
 22. The load control device of claim 16, wherein the digital processing device comprises a microprocessor.
 23. The load control device of claim 15, wherein the bidirectional semiconductor switch comprises first and second switching transistors coupled in anti-series connection.
 24. The load control device of claim 23, wherein the first and second switching transistors comprise first and second FETs.
 25. The load control device of claim 24, wherein the control circuit renders both the first and second FETs conductive and non-conductive at the same times each half-cycle to control the amount of power delivered to the load.
 26. The load control device of claim 23, wherein the first and second switching transistors comprise first and second IGBTs.
 27. The load control device of claim 26, wherein the control circuit renders both the first and second FETs conductive and non-conductive at the same times each half-cycle to control the amount of power delivered to the load.
 28. The load control device of claim 15, wherein the bidirectional semiconductor switch comprises a thyristor. 